With so many mods having been played around, it is hard to just pick a favorite one to go with. So, as with all the others, I will list my top 3.
#3: Better Dungeons (AKA Chocolatecraft). This mod adds a series of awesome structures filled with custom mobs that are quite difficult, bosses that are engaging and fun to fight, and unique items that make it worthwhile. The hidden dungeons underground are always a treat, but the real stars of this mod castles generated overworld, which are so cool that it makes you want to live in them after clearing out all of the mobs. This mod gives you plenty to explore, and while wandering through the world, you'll be awe-struck at some of the things you see. Then, terrified of the monsters living inside. This mod can stand alone, and much like twilight forest, I'd suggest it being a standard for most servers.
#2: Rotarycraft. This mod is hard, but boy does it look awesome. The idea of making and transferring mechanical power with rotating axles is awesome, and Rotarycraft adds so many things to do with that power that, arguably, you could run it as a stand alone mod. I wish it had brass coloring, so I could go with the whole steampunk theme. Anyway, by "hard" I mean that, to use this mod, you'll probably have a calculator and a pen/paper next to you to do math while build infrastructure. The power system, while simple to understand, is difficult to handle. My biggest complaint about the mod is that it is unwieldy with how the power works, in the sense that combining and modifying power is cumbersome (most efficient method is to convert to mostly torques with gearboxes which require lubricant, combine in junction which requires speed matching and only combines torques, then convert back to necessary specifications for machines). Second complaint is that it is a bit expensive.
#1: Thaumcraft. This is the mod that, no matter what pack I make or what server I play, always ends up being my main mod. The mod is difficult, and again it is so comprehensive it can be a standalone. The mechanics are fun to explore, discovering research is addictive, and doing so grants me exclusive access to some of the coolest tools and devices any mod has. Many servers have a "thaum expert", and I like being in that role. The difficulty of the mod is a bit of a turn off, and it is for this reason that I liked thaumcraft 3 better than thaumcraft 4, but thaum 4 adds so much more to compensate that it is still awesome. Either way, Thaumcraft feels like an expansion to the entire world of minecraft, instead of just additions like most mods do.
