Combiners in general.
I have POTP Volcanicus and Abominus (and some assorted limbs/torso), SS Devastator, and the KO NBK Devastator. They all stay in their individual modes since I like messing with the individual robots so much and combining/separating them takes quite a while.

 

When the Ark finally comes out in Kingdom, It will never be transformed in Robot mode. And if it is packaged in robot mode, it will be immediately switched to ship mode.

 

I got him recently and, aside from messing with his silencer/cannon configurations, I haven't transformed him at all yet, because I've read/heard bad things.

I will likely give it a go at some point with a good instruction video. I'll probably leave it like that for a while then change it back, but I think he may remain in robot mode after that.

I'm not in much of a rush there as the stuff you can do with the robot and weapon configurations is a lot of fun.

Assuming my experience is the same as everyone else, of course.

Also ER Doubledealer is one of my favourite transformers. I got him months ago. I converted him shortly after buying. Robot to vehicle transformation- fine. Then transformation to bird mode... and while most of the transformation was easy enough the leg transformation was a pain.

I haven't converted him back yet, as I'm reluctant to go through it again, although it might be easier in the other direction. I Iill do soon, but then I think I'll confining him to robot and vehicle modes. Not a big deal. While I think the bird has a lot of character it is probably the weakest of the three modes.

 

I've transformed MP 36 several times and don't find it that bad compared to X-Transbots Apollyon. I'll never ever ever try that again! MPM 7 is also a nightmare to transform. Also will never transform Earthrise Quintesson Judge.

 

The Alternity Seeker mold. What a fiddly, finicky, detach every ball joint, mess.

 

Huh. My copy of SS86 Hot Rod is one of the sturdiest feeling figures I've handled in recent memory. Transformed him at least half a dozen times with no feeling anything bad was going to happen.

It's also super, super fun to do.

 

Of what I currently own, I dont think I'm ever taking apart Warbotron's Computron ever again.

 

Oh, I almost forgot: The King of All great figures I like but WILL NEVER transform again is MP Hound. He actually has one of the most fun and satisfying transformation too. Design and Engineering perfection. Unfortunately, the manufacturer used a brittle and unstable plastic resulting in damage from handling or mere contact with air. It’s the Schrödinger's cat of Transformers.

 

Alternators Wheeljack/Grimlock. Just... no.

 

I couldn't think of a figure I felt this way about until you mentioned this. Transforming the arms is a nightmare!

 

Every single Titan class figure. Metroplex, Fortress Maximus, Devastator, Predaking, Trypticon, Skorponok, Omega Supreme. Every single one. They are all just so big and clunky and none of them have what I would call "fun" transformations. Both Devastator and Predaking seem to have made sacrifices just to accommodate the sheer mass of the figures. I love the figures as show pieces but I never think to grab them to mess with them or pose them or transform them. To me they are effectively statues. If they were Commander Class size or with Predaking and Devastator to be CW compatible even if they were a bit chunkier, I think I would be happier with them.

However, This will not stop me from getting the rumored Ark/Last Autobot Titan Figure.

 

transcraft bumblebee

 

Prime Hades Megatron.
Transformation to alt mode is useless, feels incomplete, and is a pain.

 

Majority of movie figures, both regular and studio series

 

Yet another Megatron to add to the growing trend: the MPM. That's a really complicated, awkward, unwieldy transformation and a nightmare to get everything to stay together for something that turns into, and I appreciate it for what it is but let's face it, a pile of shit.

I'd also nominate any G1 version of Megatron, Soundwave and Shockwave because their alt modes are so uninspiring, and look weird next to everything else. I'll take even the crappiest Cybertronian jet or tank over that, I just don't care to transform them if it's not a vehicle or an animal for some reason

 

My TFCC Chromedome that uses the Prime Deluxe Wheeljack mold. It's a beautiful robot & car, but something about the mold doesn't feel nice to transform. And I know it's not this specific copy since I've messed with a buddies Dark Energon Wheeljack & I felt just as weird. I guess the mold just doesn't jive well with me. ^^;

 

07 Protoform Starscream... honestly even though he survived so long and survived a kid me, I'm worried if I sneeze near him it would be like Thanos snapping him out of existence

 

Earthrise Datsuns - I already broke my smokescreen, and the other two are hard to get exclusives, like hell I'm ever-transforming them. Same with 86 jazz to a lesser extent.

SXS Hot Flame - Super fiddly, weak joint plastic, and weak paint I don't want to scuff.

 

Just like in the show...

 

Like someone else pointed out SSX Rodimus, but also MP Magnus fiddly pain in the butt, Zeta Brutics same reason also the individual bots suck anyways, anything Fanstoys (Granted I only own Hotrod), UT Predaking, and finally Naval Commander because of all the pin issues.