Vanilla+ has slim to no lag issues. It's only because it is one of the lite packs that FTB has to offer
Depends on what your into. I might be slightly biased but i would say galactic science. Also has some nice updates and plans for it.
I personally love The dark trilogy, it a mix of fun and difficulty. The pack forces you to use ALL the Mods in it EG. to get an angel ring you need to do both IC2 and Thaumcraft, you need some plates from IC2 and a thaumic harness from Thaumcraft.
