This is going to be fiction based since that's primarily what I enjoy about the hobby and the toy collecting (Especially now that I'm loaded up on toys) is more a reflection of that. Especially since they quit doing fiction for Generations.
G1 - I'm not even sure how to answer this. As much as I deride the fandoms over-focus on G1, it's that way for a reason. It's foundational. It's harder to find stuff that doesn't tie to G1 in some way. One of my favorite things about G1 though is that it was a kids cartoon and comic from the 80's, so it was allowed to have fun and do some really wild stuff that nobody had to put thought into and it results in one of my favorite aspects of Transformers: that they're basically space elves. The ark takes a 4 million year nap and when they wake up Cybertron is basically the same as it was 4 million years ago. And the war existed for millions of years before that! There's a phenomena as you get older where time appears to move faster. (I've definitely been noticing it recently. We're nearly two months into 2024 already, what the hell?!?) and adding immortality to that idea makes me often wonder what it'd be like to live at Cybertronian scale, being functionally immortal and basically immune to the ravages of time so long as you stay in good repair. Would you wake up one day (or pop out of a sleep mode or whatever) look at the calendar and go "Damn, where'd the last aeon go?"
G1 (Cartoon) - Iconic voice cast. God damn. Cullen and Welker at the top of their game and good lord that Starscream. RIP Chris Latta.
G1 (Comic) - Being able to exist well past the cartoon meant they got to highlight a bunch of characters who otherwise would have been relegated to obscurity. Nightbeat, Thunderwing and Bludgeon all owe their popularity to this comic lasting long enough for them to show up and then having good stories written about them.
G2 - Jhiaxus. God I love Jhiaxus. I mean, he sucks, but that's just because he's a great villain. Dude who puts on airs of being evolved beyond the Autobot/Decepticon war, until the combined forces of Megatron and Optimus bring him back down to their level where he completely loses it. I miss the galactic despot Jhiaxus, but as long as Megatron is around, like most Decepticon leaders, he gets relegated to mad scientist.
Beast Wars - I feel like sparks were an incredibly important addition. The fact that they basically became the default speaks volumes to how it shaped the franchise. Giving Transformers a soul, but in keeping with their mechanical nature, making it a physical object is fun and offers a lot of interesting storytelling ideas, and
when properly utilized can be used to give finality to death that was lacking in the franchise before. Not that it stops anyone from bringing back the dead.
Beast Machines - Coupled with Dreamwave's idea of having some of the gimmick lines from G1 lay the groundwork for the Beast Wars era, I'm endlessly fascinated with the idea of technorganic bodies being the future of the Cybertronian race, as if it was the destiny lying in wait for them after they fulfilled their duty in killing Unicron. Still basically the canonical ending to G1 for me even if the execution wasn't great.
Beast Wars Neo - Big Convoy was fantastic, especially when thrust into the role of mentor for a bunch of rookies, his matrix making him spew out tidbits of wisdom that he found embarrassing to be saying aloud. Just a real "Wolverine is the headmaster now" vibe.
RiD - While the image of Optimus Prime with missile batteries on his shoulders is what really got me back into Transformers as an adult, Sky-Byte is the thing from RiD that has stuck with me the most. I love Sky-Byte.
Dreamwave - As I said, the idea of taking the weird gimmicks from the 80's and turning them into the bridge technology between G1 style bots and the smaller downsized Beast Wars bots was a *chefkiss* idea.
Armada - I love me some micro transformers and making them into a third faction of Transformers was a neat idea.
Energon - Probably the most we'll ever see of two planet sized entities beating the holy hell out of each other.
Cybertron - The colonies! Love the work laid here that paid off in spades later in IDW. A planet obsessed with racing actually makes a lot of sense for a race of beings who can turn into cars.
IDW Phase 1 - Just what a breath of fresh air this was despite being a remix of G1. Dropping the 4 million year nap and changing the war to an active conflict and Earth to just some backwater that happens to become the most important battlefield was great, and Megatron's entrance into the story STILL sticks in my mind to this day. Also the complete lack of a toy tie-in meant pretty much whoever the writers wanted could show up. Nightbeat and Hot Rod effectively being second wave characters is wild.
Bayverse - Barricade was cool, and there's a good chance if these movies hadn't been the successes they were, that the franchise wouldn't be where it is today.
Animated - Just a love letter to the fandom, top to bottom. And the best part is the references and homages never got in the way of the incredibly storytelling or Animated being it's own thing in the end. Just the franchise firing on all cylinders. There's no one element that stands out to me, it's all gravy.
War for Cybertron/Fall of Cybertron - While the aesthetic isn't something I'd want for everything, it fits the world of a videogame perfectly with these super chonky robots and this giant, expansive Cybertron. Kudos to whoever decided to not put those games on Earth.
Prime - This is probably the best Transformers ever did with the idea that they're supposed to be robots in disguise - the world ain't supposed to know about them and even Megatron wants to keep a low profile. Also probably the best Welker has sounded as Megatron since G1.
IDW Phase 2 - Probably my all-time favorite bit of Transformers fiction. Post-war, RiD going full on political intrigue on a tumultuous Cybertron while MTMTE is doing wacky space adventures was everything I needed, and when Dark Cybertron flipped the script with Autobot Megatron, introducing the colonies and Prime struggling to find his place without a war to fight, the books truly became something special. These books did so much for expanding the fiction around Transformer culture, politics, biology and on and on and on. It all comes together into something truly special It's not just my favorite Transformers comic run, it's one of my favorite science fiction comic runs of all time.
RID '15 - I actually liked the Decepticon designs, though I wish they had some sort of explanation for it. Also credit to Sideswipe for being the first person in history to get tired of Prime's "Grandpa Wartimes" shtick.
Prime Wars Trilogy - Megatron was pretty great in this as an old curmudgeon the heroes have to convince to come out of retirement to aide them.
IDW Phase 3 - More misses than hits, but there was a moment there where it was cool to see all of these properties mashed together. Sadly it went off the rails incredibly fast, though Aileron and Arcee made a cute couple.
Cyberverse - Letting the hot war between the Bots and Cons turn into a cold war with a segregated Cybertron was a cool idea. Also Clobber is a sweetheart and I'm so glad they let her be her own thing and not just a renamed Lugnut.
BB - Shatter is probably the first good live action Transformer villain. Also John Cena asking the question every human should always ask when dealing with Decepticons.
War for Cybertron Trilogy - That part where Deseeus starts cutting off their faces until they reach consensus was legitimately creepy and had they not JUST introduced that character, would have been a major shock moment.
IDW2 - The mentor system is up there with sparks in the pantheon of ideas I hope becomes a common occurrence. Giving Transformers something approaching the idea of family and parenthood without just making it those things (because, obviously, it wouldn't make sense) provides for so many storytelling ideas. It kills me IDW2 only lasted a few short years.
Earthspark - God damn I love the Terrans and I'm going to be sad if they vanish from the franchise after Earthspark.
I probably missed some stuff, but yeah.
