From hardest to simplest, I'd rate them this way:
TE2 - has to be fully trimmed, painted, and the parts are asymmetrical. Also requires strips method on arms/legs.
AP - same as above, but at least you don't have to paint
RT - fully trimmed, parts are made to be easily assembled. But there is more adjusting due to split back
FX - easiest. Can buy pre-assembled with strapping too
SDS - comes pre-assembled with straps