BoP if you want more biomes that are cool
Blood Magic if you are awesome and want to sacrifice goats for the love of satan and magic
Highlands if you want more biomes that are cool, and it's also compatible with ATG and BoP
Lycanites Mobs if you are a sadist and want to die. Note: raptors are super laggy, and kobolds are super obnoxious
MeteorCraft if you like stuff exploding, also free materials
Special Mobs for the same reason as LycanitesMobs, though I prefer Special Mobs. IMO it fits more with MC, Lycanites is a little too HD for my taste.
Thaumic Tinkerer always, because it's awesome.
And Opis, Zans, Reis, and JourneyMap are all your personal preference. I prefer Reis and opis, mostly Opis because Reis likes to scream in chat the IP of the server I play on, which is a problem