I can't stand Tekkit, frankly. There's just too much stuff that serves no real purpose aside from automation; likewise with the full Redpower suite, even. I'm all for doing less work but it just feels bloated as hell, I like vanilla minecraft.
I personally only use a fairly minimal suite of mods:
- Smart Moving, because new animations and easier to get around sans stack of dirt or ladders.
- Redpower Wiring & Logic (+ Microblocks), allows some new ways of building and easier wiring, plus all the common gates you'd have to build by hand are single blocks using the Logic module.
- Inficraft Blocks, because the new stone tiles and stuff from chiselling are lovey.
- Flora & Soma, because BLUEBERRIES and new trees and stuff. What's not to like?
- Backpacks, to allow me to cart more stuff around and organize myself better.
- Not Enough Items (NEI), because I dislike TooManyItems' look. NEI fits in better and provides the same or better functionality.
That's about it, aside from dependencies (Modloader, 4096fix, Forge 3.1.2.90, Audiomod etc the usual). I launch this through MultiMC so it manages the mods for me, saves on effort installing them all every time. If I really had to cut it down to one of these mods it'd be Smart Moving hands down.
Used to use Mo' Creatures but they all had no real purpose. Horses and stuff didn't even drop leather or anything and the cat beds and fishbowls were just horrid. Love the idea, and the world does feel genuinely quite empty without it, but it feels a bit out of place sometimes.