I'm personally a fan of Feed the Beast: Infinity Evolved. It's massive, including over 175 mods, and one of the major downsides is that it takes forever to load the game on startup. In my opinion however, it's absolutely worth it if you love being able to discover new things all the time. Also if you love making automated systems.
EDIT: My favourite mods in that mod pack are Thaumcraft and Botania. Agricraft and/or Forestry are also nice to overhaul the agriculture part of the game.