I already have Mo' Creatures
Here's my list of mods that I use regularly :
1. Mo' Creatures
2. Rei's Minimap
3. TacoShell (cute little inside joke mod from a friend)
4. MultiMine (blocks keep damage so it's a little more realistic)
5. Buried Treasure (chests hidden in deserts/beaches {only in sand})
6. Spawner GUI (changes the monsters that spawn from spawners)
7. Armor Stand/Shelf mod (aesthetic mods, no real affect on gameplay)
8. Timber! mod (fall trees quicker)
9. Somnia (plants grow at night if they have adequate lighting)
10. Sandstone tools (gives a decent use for sandstone finally other than building)
11. Xie's Farming mod (avocado/orange/lemon/apple trees and a bunch of vegetables to grow)
That's pretty much the only ones I use regularly, I also use toomanyitems on occasion when I want to mess around and every now and then I get the urge to use Equivalent exchange and really cause some havoc in my world, gotta love that "Destruction Catalyst" 16 rows of 3x3 walls disappear in an instant!