Depends on what you want.
If we're talking CHUG scale and affordable, Siege is a great option. Articulated well, nice robot mode, good vehicle mode and a good size. There is the Classics mold which is old as hell now but is still decent.
If you want something different, try tracking down the Deluxe Armada "Bendy" Prime. He should be relatively affordable and he's very nicely poseable. Also he looks pretty nice (if you like the design) and he comes with a Targetmaster Minicon.
If you want to go MP then either track down MP-10 (he's dated but still nice) or if you don't mind 3P stuff then Magic Square Light of Freedom (he's not as detailed as MP-10 but he still looks good and his engineering is superior).