I'd honestly have to say I'm most annoyed with combiners in general. Sure, it's all the rage and everyone seems to go for it, but does everything need to friggin' combine??? I have very few of the A/E/C figures because the gimmick was over-done and took away from what could have been good figures (here's looking at you Grimlock+Swoop=POS 'thing'). More recently, RotF Prime/Jetfire was pointless. The toys don't work well together and make a pretty clunky statue when combined.
I have to go with Armada Prime. Love the idea of a self-transforming trailer . . . provided that your floor or table is made of the right type of material, that is perfectly level and no one breathes during the 14 minutes it takes to transform into pants mode.
My most annoying gimmick would have to be automorph. I don't own any version of Movie Blackout, but it still sucks on other figures. The two that drive me the most crazy is the Concept Camaro BB's hood automorph and Brawl's entire foward upper body. I always froget how to transform BB's hood and fear I'm going to break it, while I've already broken the gear system on one tread for Brawl (it doesn't go all the way up anymore) and the head NEVER sits right. Second place has to go to how badly electronics can limit figures (only reason Automorph beats this is because Ani. Megatron ROCKS with his evil laugh) He has a "spinning punch" gimmick that unlocks the waist and causes the upper body to rotate counter-clockwise like he was throwing a punch, only I've never seen an example of where this actually works on that mold. It's also a pain in the ass to reset to get the toy less-floppy.
I was going to post the Barricade thing, you totally read my mind. Another annoying one ... I AM OPTIMUS PRIME!!
Supreme Devastator Mixmaster and Rotf Megatron cannon sound. I can't even move his arm because of it, and I got so sick of Mixmaster I took out the batteries.
yeah thats pretty lame, the button usually takes alot of force and the moth only opens a very small amount it killed most of the individual transformations. I do like the energon fatty prime though....at least his combination was made up of non transforming drones so "no bots were harmed in the making of this combined mode"
TFTM Barricade's punching arm Armada Sideswipe/Runamuck's punching arms TFA Voyager's Optimus Prime's spinning waist
Imo, the only sound that should come out of a Transformer when transforming it should be "clicking" from pieces coming out of and going into place.
For most annoying, hands down... any sound effect that goes off with movement. And by that I mean Energon Ironhide and Energon Scorponok. Really, do I need to hear those piston sound effects EVERY TIME YOUR TAIL TWITCHES?!
Bad automorph. When it's minor and unobtrusive (deluxe brawl's is quite nice) it's nifty! But when it's not, like in TFA OP or Starscream, it makes transformation a total chore. Both those figures are better if you just remove the gears and transform them by hand. Twitchy sound effects may be annoying, but bad automorph breaks the toy's basic reason for being; transforming from one mode to another. That has to make it the most worst possible gimmick a transformer can have. edit: When I first saw this thread, I thought it said most annoying GRIMLOCK.
I generally enjoy most gimmicks, so my answer isn't exactly in the vein of the other replies here...when they make up some weird configuration that's really just a fan mode and claim the toy is a triple changer. ROTF Mixmaster and Soundwave being perfect examples of this (although I know Mixmaster did the turret thing in the movie, the toy's representation of it is really just a pulled apart robot crawling around).