The th400 is the strongest tranny out of the 3. Bulletproof. Mmm, that's nice.
The th350 is pretty hardy, but not like the th400, and the 700r4 is nice for highway driving because of the overdrive, but isn't as strong.
The NP203 t-case is a full-time 4wd case.
The NP205 is a bullet proof t-case and is gear-driven.
The NP208 is an okay case, but is chain-driven.
I'd recommend the TH350-NP205 or the TH400-NP208, for strength reasons mostly.
BTW:
My '85 GMC is a TH400 (beefed up a bit) with a NP208. I liked the combo alot, and with the torque of the modded 350, it went anywhere I told it to without ever breaking.
My brother's '76 Chevy is a TH350 (beefed up) with a NP203 (changed to part time 4wd so it has a 2-LO option as well :D ). The tranny couldn't handle his driving, and is in piss poor shape now, leaking from everywhere. It's not a bad tranny, just couldn't handle him.