Very tough. I'm sure my top 5 would change depending on what I'm in the mood for.
But just scanning the figures I have out at the moment:
T-600 endoskeleton (amazing engineering, LEDs are fantastic, design is cool albeit not quite what we were led to expect)
DX Joker (all the obsession kinda annoys me but it is undeniably fantastic)
Predator 2 hunter (not the newest or most sophisticated but I'd still buy it if it were a new figure released today)
John Connor version 1 (decent likeness, outstanding outfit design and realisation - give him the gun and base from V2 and its a beautiful display piece)
T2 T-800 model 101 (don't have it yet, some pics are better than others but I believe this one will not disappoint)