I have found, that Universe Nemesis Prime was a complete pain to transform. The instructions were lame no language at all ones, and they started from Beast mode, yet he was boxed in robot mode.... Never successfully got him into beast mode until I just recently acquired a korean Big Convoy with instructions and language (although Korean...). even then, instructions were from beast mode, but robot mode in box, still at least they worked better and I finally did it.

Now I just find that due to mould changes, parts of Nemesis naturally fall off and he just doesn't fit together well, indicating that when I tried TF-ing him , I was pretty close, but poor quality stopped (put off more like it) me from going any further...

Forming Omega Prime (RID) was a bit of a bugger too, i remember spending at least an hour on it and working up quite a sweat in the process, although it was christmas time.

Easiest? Battlechargers or Cosmos perhaps?

I find MP Grimbo quite easy, its close enough to the original to give me little tingles, with funkiness happening on the legs. Never found it all that hard once I had a good look at the way the tail folds away.