I'm quite fond of ICBM and the way it reinvigorates PvP. It basically can turn two factions' war into a cold one. MFFS also adds great defense from mobs, players and nukes. This version is far superior to the IC2 version. You can do so much more. These two mods are, quite simply, fun. I also enjoy MineChem, dispite it being a lot of standing still. You can create quite complex systems. Mekanism adds so much content and the things that it shares with IC2 are played in a lot more fun a way. IC2's ridiculous need for lots of materials and long crafting recipes can be off-putting, especially early game. This is true even before the new updates. Resonant Induction's power and item transport system is visually striking and it surports Multiparts we'll. Dark's mods have a few great additions, but they're mostly so-so. I've never truly used Atomic Science. Then there's the great mods with compatibility (Galacticraft, MPS, MFR, Railcraft) they all add so many great features.Galacticraft's planets (despite being next to pointless) can be quite fun to explore and a space race can only improve the Cold War element. To compare IC2 to the UE family is unfair as IC2 is one (somewhat dated) mod and UE has just so much more to it. You can cover most of IC2's features with these mods and a lot more. But, TBH, the mods are so much better when used in conjunction with Buildcraft and IC2 power, the mods appear to of been designed to work with the pair of them and they do so well. I play with a very large personal modpack with 200+ mods (surprisingly not that buggy - but still could do with some narrowing down) with all of these, and in every new world I like to create a lot of Buildcraft power to power Mekanism machines. I say UE does cover next to all bases, but some mods do things better (especially textures) and I don't know how well they stand by themselves. Reading this thread it's clear how important Mekanism and Atomic Science are, my fave route mods are probably still ICBM, MFFS and Galacticraft (what can I say, I'm very competitive), but those mods are improved and complimented by so many others.
Edit: Just worth saying, install ICBM and revel in how awesome the explosions and missiles are.