Personally I think that Mekanism and Chisel would be two good mods to add to tekkit. I've been watching yogscast lately and they seem to use those mods fairly often and I tried them for myself on ftb (and later ended up deleting it because of computer memory loss) and I honestly think they would be good to add into the modpack, another good mod to possibly add would be "biomes a plenty" just to add a little bit of variety to the plain vanilla biomes. DISCLAIMER- I did not make any of these mods and although i cant name the authors because I don't know who made them, I must say to you mod creators AMAZING JOB
reply below on what mods you would like to see added into the Tekkit modpack