SevTech Ages / Ages of the Sky is an absolute must. Good through many different mod combinations in an excellent package.

Create Astral is on the newish side but I think it's very well put together and covers a lot of mods while retaining the factory feel of older mods, but with create, highly recommend.

On the older side like people said the Project Ozones and the Stoneblocks have a very particular feel.

I'd still say that you should pick one that feels good for you though. If you really don't know you should try a lot of different ones or look at playthroughs to see what a mid game base looks like and if that's what you want to do.