I found the Suburu Alt/BT pretty decent to transform, once you've done it once, it sticks. Grimlock/WJ on the other hand are murder.
Agree with Gok about Big Convoy/Universe Nemesis Prime and DotM Prime being pains in the butt, I would like to add Sentinel prime as another large and frustrating figure to transform. I always found BW TM2 Megatron a bit of a pain to transform, but he always stays in Dragon mode for me.
There are a lot of figures that would be very difficult to transform if you didnt have the instructions or previous knowledge of what each mode/s is meant to look like. RID and the movie figures jump to mind, as does BW Depth Charge and Tigerhawk, context is key with those figures.
WFC Bumblebee and even Prime were pains, but not to the movieverse figures levels!
Looking For: Wreckers Saga TPB Collection (with Requiem)