I'll throw in another vote for MP Megs. Not as tricky / fragile as some would have you believe, but definitely not one to be rushed.

RotF leader Prime is fiddly into truck mode. Not so much the other way.

The arms on Alt Wheeljack... ouch... rarely do I manage that without either the arms or the doors (or both) popping off.

SWTF Death Star. Too many finicky panels.

TFTM deluxe Bumblebee - dumb arms that won't go into alt mode easily  

 

07 Movie Concept 'Bee.

Mainly because his automorph got misaligned and he was stuck in robot mode for months until I managed to force all the parts together.

For one who was difficult without defects, Movie Ironhide. ESPECIALLY into truck mode.

 

At times, i just feel like throwing that thing into a wall.

 

Long list for me:
Alternators
Universe Ironhide/Ratchet, and Galvatron
Ultimate BB
Masterpiece Prime

 

The one that sticks out in my mind is Beast Wars Rampage. Simple transformation scheme, given. However, his treads are the spawn of satan and do not cooperate at all  

ROTF Leader Prime can be difficult, and I think is one of the most complicated transformers I've owned.

 

I transformed him once to gun mode. Took me two evenings, maybe 2h30 total.
never changed him back!

After this one, all others are fairly easy!

Just like rubic cubes to me!

 

^this, stupid thing won't go back into jet mode properly, MP megs is simple after reading the instructions once, the Mustang alt is a pain in the ass too, the doors do not like to pass around the hood to get the arms into either mode

 

Alternator Shockwave

Spent serious time trying to transform without the arms popping off or the hoodpiece over the head popping off.

 

BW Neo Dead End.... Transformed him once, never managed to get him back!

also R.I.D Megatron is quite confusing for some of his modes, better to just make some up!

 

Really? I can kind of agree with Smokecreen (only about his transformation to Alt mode)...but Sideswipe?? He has got to be the easiest Alternator ever. The only Alt I would say might be even more simple to transform is Alternator Hound.

If you are talking about his arms popping off at the shoulders, I believe that is a defect for the mold. The right arm on my Alt Jazz does the samething. I have to be very gentle when moving it, otherwise it will pop right off.

 

Here are a few of the tougher ones:
BT/Alt Mustang mold
MP Megatron
Megabolt Megatron
Big Convoy
THS 02 Convoy

 

The Universe Ironhide mode.

Holy crap this guy will never be in SUV mode again.

 

Alternator Optimus!

I still cannot get him back into Alt Mode.

 

MP megs.

what makes him difficult is that the skeleton frame that all the parts are attached to is difficult to manipulate easily during transformation, bits seem to get in the way of each other and of course there is a danger of breakage as some of those hinges seem very thin.

of the standard toys for kids that i have id say that the prowl/smokescreen universe mould is annoying, well, the door/wings parts, they are very fiddly and i hate the fact its easier to remove them to transform the toy and then just put them on in place. ok, its not a biggie but theres something about the frustrating aspect of that toy that stops it being fun, TF's should be simple an intuitive you shouldnt have to grind your teeth and get all antzy playing with a toy for 5 year olds!

 

Um, either Movie 1 Leader megs (i cant get it past step 6) or ROTF Chromia

 

From what i have seen, Alt Grimlock (or wheeljack), Most of the MP's, I haven't seen it in a while, but i think RID ultra magnus was kinda hard. there are tons that are fairly hard.


I LOVE HARD TO TRANSFORM TRANSFORMERS

 

I got them into Omega Prime, however they have BEEN that way for about 3 years. I don't think I could remember how to get them back.

My votes are for MP Megs and Alt Mustangs as well.

 

Chromia's pretty simple actually. Probably the simplest Deluxe I've seen in the ROTF series, right next to Smokescreen.

 

As a few have said, MP Megatron isn't as hard to transform as others make it out to be. He seems fragile, but will be fine if you take your time. Kinda surprised someone said MP Prime. He seems to be the simplest of the MPs.

Been a while since I transformed any of my Alts, but it was Wheeljack that gave me the most problems. I gave up on transforming Alts like Wheeljack and Jazz without having parts pop off. If that happens I just reattach the piece after getting the rest transformed.

When I was young, my neighbor couldn't figure out how to get his G1 Jetfire transformed properly into any form. It was just a jumble of robot and jet parts. He seemed pretty upset when I was able to transform it to all three modes (didn't even have a Jetfire of my own yet).

 

I've got Wheeljack and Grimlock. I generally try to keep one of them in car mode at all times so I can look at the shoulders and such under the car.

BT Skids gives me problems frequently.

The one I actually have the most issues with (so far) is BT/KP Prime. The transformation it's self isn't that tough but it seems to require more force than I can possibly generate without snapping something to get the legs to go back in and the shoulders in for truck mode. These joints require significant pressure and there's nothing solid to push against to generate this pressure.