Alright, after much deliberation I think I've got my ten. Five of the worst, and five of the best.
Anyway, the worst 5, in no particular order
Alternators Grimlock. As previously mentioned, the Alternators had their share of growing pains to them and dealing with issues inherent to being a fully licensed vehicle first and a robot (sometimes distantly) second. But of all the ones I handle, none are as joyless as Grimlock. Parts pop off easily, things getting in their own way, that infamous Not-A-Waist waist swivel that always felt like it took excessive force to budge. There are ideas here, but none of the execution works for me.
Blackout (Movie 2007 Voyager). The '07 toys were given the unenviable task of trying to translate the incredibly complex Bay designs into Cybertron era engineering and while they all had their various hits and misses, they all had to deal with a big caveat: include Automorph gimmicks. Self transformation gimmicks don't always land, and Blackout just falters because of this. The "pull lower half down and peg into place also flips the torso around" sounds simple enough, but in execution becomes a parade of repeated attempts to get it to line up as you figure out what exact motion those gears and panels want from you. A few extra parts that don't like to lock in just make him not nearly as fun as it could be.
Human Alliance Leadfoot. Unlike the previous mentions, things move easily on Leadfoot. The problem is they never STOP moving. There's just no point that the parts ever lock in or present themselves in a way that makes sense or even comes across as aehsetically pleasing. He's a rare kind of Transformer toy, the kind that feels like a design was conceptualized, but then never finished. A bizarre first draft that never got refinement.
Classics/Universe 2008 Galvatron. You could probably fill an entire list of your own with the things Universe '08 did wrong by deciding it should be the "replacement" for complicated toy designs while Animated handled "simpler" ones. But of its many missteps, Galvatron proudly comes forward, and promptly trips onto his face. Parts pop off if you so much as give him a stern look, kibble bumps around and vyes for space, and bizarre phantom steps from its days as a Voyager concept stick around to just make you scratch your head and ask "why".
Revenge of the Fallen, um, The Fallen (Voyager). RotF's troubled production certainly turned out a terrible movie, but the toys had to deal with constant rewrites and conceptual dead ends. But the film's titular villain falls flat-- literally. A robot design with no alternate mode in mind, Hasbro had even less to work with than the prior film's Megatron, and so worked out how best to make something out of nothing. And to that end, The Fallen isn't a Transformer. That the steps to go between modes aren't fun is reason enough the other four got on this list. The Fallen offers no reason to go between either mode in the first place. His robot mode is, when "properly" finished, an immobile statue that can barely stand up due to those goofy spring-loaded feet. And his "jet" mode is just mishmash of alien robot limbs making no attempt to hide what they are in the slightest or form a shape outside of "lump". You could argue all these sins are true of its modern Studio Series update, but at very least that one can function in its robot mode-- as good of a toy of an inherent janky design as one might hope to get. I just find no joy in The Fallen's intial outting.
But that's all the negative stuff. Time for my all time greats.
G1 Sludge. Yeah, this list actually was me spanning the entire franchise, it just happened that all the bad stuff was in one concentrated timeframe to me! But anyway, Sludge I feel is one of G1's strongest for just solid play. Simple, but in a way that flows well. The back splitting into robot mode "wings", the timeless 180 degree waist turn, even things as simple as his rear dino legs being able to be pushed in closer to his calves for robot mode to clean up his look. They put a lot of care into this design and he's easily the best of the Dinobots... sorry Grimlock!
Robomasters Rijie. That's the flipchanger Formula 1 car mold. A well done autotransform can really be enjoyable, and I think Mirage gets to rep the group best. Everything gets accomplished in one simple step between modes, really suiting a fast race car Transformer. The way the weapon storage splits his weapon into both pistol and stock and stores away so neatly just further make him a joy. If you want a different example but equally good, it was between him and his Beast Wars brethren, Insecticon.
Universe Reptilion. I will always tease the common nature of "stand up, switch heads" among beast Transformers, even if it makes perfect sense from a logical standpoint. But Reptilion (and his original deco, Transmetal 2 Iganus) at least changed things up enough to make me smile. The frilled lizard has, on paper, no surprises: those limbs are still limbs after the change... but the way the design flips things around (pun not intended) to become a warrior wearing a tattered battle skirt and sporting one nasty whiplike ponytail really give him so much charm and uniqueness. He could have been as bland and forgettable as his original flipchanger self, but the way they handled the main torso really made me enjoy just how much personality the toy wound up having.
Unite Warriors Devastator. This isn't about the indivual Constructicons, or their transformations. No, the comibination to and from the mighty super robot is one of the most fun I've ever enjoyed with a toy-- I was putting together and taking apart Devastator over and over when he first arrived. The sheer solid stability when you're finished almost seems impossible to believe considering how many components are moved and adjusted getting there, but he pulls it off. People murmur rumors about an update for Studio Series or something, but to me the bar has been set incredibly high.
And finally
Holiday Optimus Prime/Volvo VNR Optimus Prime. This toy. This toy. Wow. Every step of the process feels just perfect. Things move intuitively, and there's so many spots when something locks in and you're just left gobsmacked at how perfectly it lined up. The grill kneecaps slotting into the calves. The robot chest pegging in perfectly onto the folded down mudflaps of the truck mode. The flip-and adjust of the wheel backpack to get it nice and close to Prime's torso. This toy is a Masterpiece on every level except name. Simply one of, maybe even the best Transformer ever made.
And some honorable mentions who were considered but ultimately had weren't on either list
Best: Beast Wars Razor Claw/Rockbuster, Beast Wars B'Boom, Cybertron Downshift, G1 Soundwave, Age of Extinction Voyager Megatron
Worst: Universe '08 Ironhide/Ratchet, the G1 Seeker, RotF Leader Jetfire, RotF Skids as well as RotF Mudflap, Universe 08 Dinobot, RotF Arcee,
