For me it was Predaking. I'm not usually a fan of orange, but it suited him well, especially with the black. He had a great, intimidating look, and in my opinion, was the best looking combiner.

My other favorite robot modes were:
Galvatron (even though the toy was not good)
Jazz
Soundwave (he made blocky work)
Trypticon
Kup (not great toy again)
Blitzwing (love his color scheme and the turret on his back)

 

I like a lot of those choices (Bruticus fan forever here but I have to agree Predaking was the most impressive looking of the combiners) and really like Soundwave too but my #1 pick?
Honestly, it's Starscream. I've always thought the Seekers are wonderfully iconic, look great with those wings on their backs, and Starscream was my favorite of all. That red, white, and blue really pops and he's just got a great looking root mode.
Slightly oddball pick? Onslaught. I just really like his color scheme.

 

That's pretty much asking about my favorite characters.
Predaking, Galvatron, Cyclonus, Ultra Magnus, Astrotrain, Mindwipe, Powerglide, Hardhead...

 

That is an incredibly broad question...

 

That's basically the same like my favorite characters:
Rodimus
Optimus
Starscream
Galvatron
Scourge
Scourge (RID)
Cyclonus
Arcee
Springer
Ultra Magnus
and more

 

Way to many to list but I'll keep it down to 5. But from different continuities and shows.

Movie Roadbuster: Single handedly my favorite bayverse design.
G1 Wheeljack: Manages to look expressive and badass even without a mouth.
WFC Breakdown: Surprisingly we never got a toy of him.
Idw Whirl: Simply awesome.
Prime Soundwave: Extremely menacing.

 

Jesus H. Christ, I can't even pick my top 10 characters, how am I supposed to pick my favorite designs?

 

Design? As in 'how it looks?"

Probably Cybertron Optimus Prime. I don't think I've seen a Transformers robot mode exceed it yet.

Although Energon Optimus combined with Wing Saber is right up there too...

 

G1 Prowl
G1 Blurr (animation model)
G1 Hot Rod (animation model and Mega SCF Rodimus figure)
G1 Cyclonus (animation model)
G1 Tracks (animation model)
G1 Dead End (toy)
RID Scourge
RID Spychangers
G1 Ironhide (animation model)
G1 Megatron (early animation model where the shadow on the underside of his nose and his cheekbones look kind of skeletal)
Robotmasters Mirage
G1 Seekers (animation model)
G1 Defensor (toy)
G1 Menasor (toy)
G1 Razorclaw (animation model)

 

Seekers in general look really cool. I also loved the designs of most of the car Autobots like Prowl, Jazz, Mirage, Sideswipe, Sunstreaker, Wheeljack and Tracks notably, the Cassette Transformers Blaster and Soundwave, and Bumblebee is adorable

 

I've always liked the color scheme on G1 Smokescreen.

 

• Sunbow G1 Optimus Prime
• Age of Extinction Lockdown
• Car Robots Gelshark and Blavk Convoy

 

Optimus Prime

 

G2 Laser Rod Optimus Prime, G2 Hero Optimus Prime, G1 Grimlock, G1 Jazz, IDW Megatron (great tank).

 

Even though the toy was a fiddly mess, I loved the look of RID Prowl/Car Robots Mach Alert.

While we're at it, RID Nemesis Prime (refuse to call him Scourge, that name is reserved for blue sky sleds) looked absolutely fantastic. Okay, basically ALL the non-BW mold characters animated in RID were awesome.

 

I love most Bay designs.
For G1, it's Skyfall.
Google Image Result for https://d1w8cc2yygc27j.cloudfront.net/-8066234483542415723/-7891779955754212004.jpg

 

Off the top of my head? Movie Bumblebee.

All the car parts contribute to the robot physique, which fits the character. It looks (at least by Movie-verse design standards) friendly and approachable, with slightly bigger eyes, the signature Bumblebee antennae, door-wings that fit the bee theme. However, it doesn't look weak, and feels a lot more like a survivor of a four million year war. The gas mask-like mouth plate, the sleek black racing stripes, and the sense of strength behind the lean build convey that, although Bumblebee is young, he's survived the war because he's a fighter. As a bonus, the design from the first three movies and TLK incorporates enough of the vehicle mode into the robot mode to give a sense of where everything goes.

 

Always been a fan of Powermaster Prime/Super Ginrai

 

I think G1 Optimus has one of the greatest robot designs ever. Big, simple, uncluttered, heroic, and great placement of parts. Even in the 'corny' cartoon, he really stands out as a quality design.

My second place would probably be G1 Soundwave. His use of colors really looks good, and he has a lot of distinctive features (while still looking uncluttered).

There are so many good ones. I'd have to give honorable mentions to G1 Megatron and Shockwave. Also, I really like Jetfire's original (toy-based) design. I honestly wish it was used in the cartoon, because IMO it looks a lot cooler than the one they used.