RID Megatron (The one in the picture above is the repaint Galvatron) is a neat figure, but is very un-transformery in his design and aesthetics. Also suffers from many of his modes being silly or unrecognizable, but he turns into the fucking Batmobile, so I won't complain.
The Autobot opposite of Sixshot was Quickswitch. Not bad, but also not as cool looking as Sixshot, so he's not as remembered. I don't think 3rd party companies have bothered with him.
OpOp is a pretty "meh" toy, honestly. It was a big deal to have a toy of that size and number of modes come out in an era when we hadn't seen anything of the sort in many years, but his colors and proportions leave a lot to be desired for me.
Secret best multi-mode Transformer: Beast Machines Scavenger (or his less garish RiD repaint).
He's so... Fiddly. There were a bazillion fan modes for him back in the day, all of them more defined than most of RiD Megatron/Galvatron's modes. Ironically his robot mode leaves much to be desired, but he can turn into a robochocobo, so all is forgiven.