If you could only have one...

[Zeryxis]

It depends on what I want to do that day. I havent delved in Thaumcraft as much as I'd like to (dont wanna muck up my place!) but

Millenaire for questing/people that arent dependant on me completely for their survivial

MineColony for when I want to play "island survivors" (*even have a seed just for it, starts with 1 tree, some grass, and 1 stand of sugarcane on another smaller island)

Mo'Creatures because I love horses and like having more variety to the local fauna. Also want a herd of zebras. yaaaaay zebras!

So overall I'd say mocreatures because ore creatures. But it depends, really.

[Garnage]

Redpower2, There just isn't much of a limit to it.

Thaumcraft is a very close second, mostly for the meteor boots and portals, not to mention seal combinations. It has a pretty solid set of end game goals though. With Redpower the limit really is just the user.

[Hushful]

EE2 for sure and redpower2 is a really close second.

[felinoel]

Definitely Millenaire, it turns Minecraft into both an Adventure game AND an RTS, I love pitted villages against each other and watching the raids to see who is victorious.

Plus it make finding a home base for everyday Minecrafting easy.

Then there are the lonebuildings, these are fun, scary too like when you are minding your business chopping down a forest when out of the blue a pack of bandits swarm you and you have to defend yourself lol

[NightKev]

Is "not Minecraft" a valid choice?

[110toby]

Equivilent exchange

What am i suposed to do with all the cobble and dirt from quarries otherwise?

[Sacrieur]

Equivilent exchange

What am i suposed to do with all the cobble and dirt from quarries otherwise?

But if you picked EE you wouldn't have quarries.

[SPGold]

Redpower 2. The only mod that allows me to accomplish all ideas in Minecraft (except for an automated building machine of sorts and on demand autocrafting but that is planned for the near future). Buildcraft a close second.