Lol, this is funny.
I found forming Omega Prime quite time consuming for the first time. I actually remember breaking out in a sweat as I did. First time that happened.
One that I found fairly tricky was Sentinel prime from Movie 3. Managed to get in in the end, but only tfed him once, mainly because of all that damn panel aligning. Really irritated me no end that did, as soon as I had one part set, the part I had previously done would pop out.
Yep, sold that sucker soon enough.