Pulling from my entire collection here:
Minicon: Deepdive, simply because he reminds me of that one GoBot I had as a kid.
Legends/Cyberverse: Until recently, I would have said LOC Starscream... but I got my hands on Cyberverse Prime Cliffjumper, and his articulation at that scale is simply amazing.
Scout / Basic: Ooh, this one's a bit of a toughie, though not as tough as Deluxe. It's a tossup between TFCybertron Armorhide, and HFTD Hubcap. Armorhide has a lot of character and his entire chest opening up into a missile bay always makes me laugh, while Hubcap turns into ZZ Top's Eliminator!
Deluxe: After a knockdown, drag-out brawl on my shelf, Generations Jazz has emerged victorious, closely followed by his buddy Warpath. However, the Classics Etc Seeker molds nearly took the title!
Voyager: And here is where the division begins in earnest. On the playability side, gotta love Vector Prime. Simple but effective transformation, and a good bit of both articulation and personality. However, moving in from the DOTM line, Skyhammer just totally dominates in both helicopter and robot modes, only losing points because the transformation has become less of a 'play' function, and more of an IQ test.
Human Alliance: Although I'd normally classify these guys as "basically just Voyagers with a human figure", Human Alliance Jazz convinced me to put him in a class by himself.
Alternators: Also a scale I'd normally classify as "Voyagers with attitude", but the Smokescreen / Silverstreak mold is and always has been a favorite.
Ultra: Actually, I don't have too many of this scale - about the only ones I do have are the Classics Storm Cloud, Silverbolt, and Onslaught, and TFCybertron Skyshadow and CD Red Alert. Of those, Onslaught is probably my favorite.
Leader: Again the vote is split - between Optimus and Optimus, and for the same reasons! In the left corner, we have TFCybertron Optimus Prime: Filled with chunky, durable goodness, a nice but simple transform, and a total beast when armed up with his trailer - or with his buddies jumping on his back for more power! In the right corner, we have ROTF Leader Class Prime - amazingly screen accurate, excellent articulation and presence, but more of a chore to transform than a delight.
Supreme: I only own one Supreme, and it's one that, if i didn't have it, would be at the top of my list in one repaint or another, and that's Unicron. I got the Armada version, and it's probably the only Armada figure I own...
And for the non-standard figures:
Masterpiece: 20th Optimus Prime, no doubt, although Grimlock comes very close. Honestly, I'm not really disappointed with any of them, but these two just rule the roost.
Crossovers: Whether Marvel, Star Wars, or Disney, my favorite is and always will be the Incredible Hulk Tank, with Captain America Humvee right behind. And why, you may ask? Because they're the only two worth a damn that aren't too cartooney.

ORIGINAL GEEWUN: If i had a G1 Jazz, G1 Bluestreak, Misfire, and Fortress Maximus, I would neither need nor want any other Geewuns. Mostly for sentimental reasons.
GoBots: I have me a Screwhead, 1983 vintage, whose paint and chrome have never chipped or flaked yet. Point of pride, that!