A lot of the "hard" mods from packs like Agrarian Skies can be tuned down to not-deadly levels and still provide incentives for you to actually give more than a cowfarm a chance. AppleCore is the coremod for Hunger Overhaul and Spice of Life, and even if you're not using them, the features it provides are amazingly informative. The Hunger/saturation HUD overlay? AMAZING.
http://www.minecraftforum.net/forum...core-an-api-for-modifying-the-food-and-hunger
It also has links to HungerOverhaul and SpiceOfLife
Hunger Overhaul can be used to make things EASIER if you'd like, and fixes a few silly bug/issue/oversights in vanilla. You can also disable basically anything in the mod if you don't like it (The starve-just-standing-there is one I usually disable. I don't like dying because I had to go AFK and forgot to log off).
Some form of backpack mod is pretty standard. So are biome mods like EBXL and BoP. NEI is always a good idea, as is WAILA.
Applied Energistics is pretty traditional anymore. So is JABBA.
It's hard to beat Bibliocraft for functionality and aesthetics.
I will recommend Botania to everyone for everything, even tech packs. It's solved so many niggling problems with all kinds of things spread through Minecraft that I haven't seen a reasonable solution for.
Buildcraft you're already including. Forestry I don't usually recommend, but if you're including it, get Binnies Mods, Magic Bees, and try to include Gendustry (even if you nerf/break its bee manipulation machines. The ability to add bees via json is really nice especially if you can get someone that's a bee nut to help you with cross-mod bee products that aren't covered. Like say Project E.)
ChickenChunks is one of the most stable chunkloading mods out there (surprisingly. I normally would say Immibis' dimensional anchors but basically every frame mod breaks them; except CB's)
Chisel is another lightweight mod that adds a LOT of aesthetic options.
One of the NPC mods (Ancient Warfare, CustomNPC's, Millénaire, Minecraft Comes Alive, etc) seems almost required for a challenge like this, just for verisimilitude.
I'm a personal fan of Ender Compass and Nether Eye (Because finding Nether strongholds is a royal PITA, and wasting Eyes of Ender on finding strongholds sucks)
EnderIO is a good "core" tech mod. If you don't have another mod (besides BC) that adds energy, fluid, and item pipes? Use EnderIO. Buildcraft's pipes really don't cut it as your "workhorse" pipes anymore. I get the whole aesthetic thing, but basically every other mod does item transport in a more usable and easily understood manner.
One of those mods that adds alternative piping is Extra Utilities. It also has a lot of fancier looking decorative blocks, one of the only non-bee, non-magical_crops ways of getting enderpearls without spawners or an Ender Ender (Drop-trap), and an AMAZING multitude of ways with which to generate power.
EnderStorage was added (in reduced form) to Vanilla for a reason.
Speaking of those resource growing mods (Sorry, any pack with bees doesn't get to call things like Magical Crops cheaty), I'm rather partial to Fluxed Crystals and Pam's Mob Drop Crops. Though there is also Magical Crops, as well as Attained Drops.
Funky Locomotion, Framez, Remain In Motion, and Truss mod are all nice frame mods (there are a couple of late-game suggestions that would require frames to implement... or possibly Archimedes Ships... though that's reasonably incompatible with moving your whole base as compared to frames)
IndustrialCraft has fallen out of favor, though I'm thinking of including it in my pack for old time's sake, even with the changes. It was my first mod, so I'm a bit of a softy for it.
InventoryTweaks (if you have the ability to make optional mods) should be an optional inclusion. Some people can't play without it, others can't stand it.
JABBA.
I'd suggest Logistics Pipes, you can't get into it until... Gold City (maybe a bit later depending on if Maul ever fleshes out the later stages of the challenge), but it's seriously awesome, more people need to be exposed to it. I actually prefer Logistics Pipes to Applied Energistics for everything except item storage.
Mekanism is another pretty good tech mod.
MrCrayfish's Furniture Mod fits well into the aesthetic category (and now that I think about it there was an agriculture/cooking mod that added a lot of kitchen appliances that actually worked. I think it was the one by the metallurgy team)
Try to include a portal mod. PortalGun used to fit here (but for some reason iChun is postponing both the "gun" mods to 1.8), Mystcraft, Enhanced Portals 3, RFTools, IndustrialCraft, EnderIO. There are options all over the place, but with the size and distances we're talking about in a R2R challenge? Some form of extremely rapid transport is nearly required.
Natura is nice for the early game. Just check out how much
@The Mobius Archives used it in his playthrough. Plus, cotton is amazingly useful. String is one of those resources that can be a bit of a pain to get unless you find a spider spawner. Also the redwoods? Epic for building treehouses! They can also be a good (or bad depending how you look at it) thing if you want to mix in the Tree Spirit Challenge.
Obsidiplates is in most of my packs, and there are a surprising number of ways to get obsidian well before you could normally mine it. Carpenter's Blocks also has a player-only pressure plate.
OpenBlocks is another mod that's in a lot of packs because of all the generally useful things in it. (It being one of the only three mods that deals with liquid XP and all)
ProgressiveAutomation is a mod you may want to look into. It provides a lot of mid/early game automation farms in an amazingly balanced way... can't recall if they all require redstone or not though.
ProjectRed is nice for the logic gates (MFR, ComputerCraft, and EnderIO can duplicate a lot of that functionality) and the marble/basalt... which you can get from Artifice as well.
Railcraft is amazing for transportation while you're locked out of the higher tier tech/magic mods in Redstone/Gold stages. Plus... Railroads are cool!
I'm liking RandomThings more and more as I play with it. For a lot of the same reasons I like Botania, EnderIO, Extra Utilities, and OpenBlocks. The "random compilation mods" are great for solving all kinds of nagging problems in places most people wouldn't have thought of. Plus the Bloodmoon, Fast Leaf Decay, and Hardcore Darkness. I personally turn off Hardcore Darkness, but at the very least? Install the leaf decay mod he made. Not having to stand around for 5 minutes waiting for leaves? AWESOME
SuperCraftingFrame Is so nice for those things you know you're going to be crafting a billion and a half of.
It's hard to beat Tinker's Construct for removing a large amount of ingame tedium regarding re-crafting tools. The smeltery is also an amazing ore-doubler before you get "proper tech" (or magic). It's one of my preferred ways to get obsidian before I get "infinite" lava (then it becomes the TE Igneous Extruder). Plus it's one of the best ore multipliers unless you've got Mekanism, Factorization, RotaryCraft, or um... I know there's another 2.5+ ore multiplier out there. I actually prefer it to Factorization's method. I speak of Fortune + Auto-smelt.
TE is in most mod packs because TE is honestly one of those mods that adds a lot of very "core" machines and features a lot of people got VERY used to having. Its fluids are fun, florbs are awesome, not many mods have a proper deployer replacement let alone a block breaker, it's got a reasonably intuitive autocrafter that is earlier game than most of the other options (at least for R2R. Though Progressive Automation, EnderIO, and AutoCrafter2000 are competitive... no idea about the crafty crate from Botania, and I'm pretty sure the transfer node with world interaction upgrades only really requires access to Lapis and Redstone). But it also is the "easy" solution... so a lot of people leave it out.
Translocators are often forgotten, but they're very nice for compact builds and that in-world crafting table on any solid block? Saved my bacon in the field and on top of mega-structures so many times!
The Vegan Option is neat, but I'm one of those players that rages at the random nature of world generation and mob spawning, so having a reasonably reliable option to getting stuff like that is something I can get behind.
WAWLA is nice if you're a heavy WAILA user, it's also one I don't see in many packs for some reason.
On the magical front? Botania (Yes I know I said that already. It's one of my favorite mods ever), Blood Magic is one of my fav's as well (though the subject matter can be a bit... squick-inducing), and if you're doing Thaumcraft? Thaumcraft Node Tracker and Thaumcraft NEI Plugin are totally must-haves. Yes I realize there's minimap mods but NodeTracker is automatic and doesn't clutter the crap out of your minimap waypoints, and yes all the recipes are in the Thaumonomicon... I still notice how much time and frustration I save with proper NEI integration; that book is a pain to navigate sometimes! And at this point I'm definitely forgetting some awesome mods... but this list is huge so... blah