Optimus Prime, Megatron, Soundwave, Starscream & Teletran 1 were my favorite voices.

I'd also go with the most distinctive and memorable acting - hence both Megatron and Prime. I'd give Megatron the edge because the voice is unique - I can't remember a voice like it in any other show or film, but it is instantly recognisable.
Most of us came to like Starscream but the same voice is heard in Reflector (though toned down) AND Cobra Commander

Frank Welker as Megatron and Peter Cullen as Optimus Prime.

Victor Caroli has an amazing voice. He was perfect as the narrator.

2nd place 3 way tie: Roger Carmel as Cyclonus and Phil Clarke as Dead End.
also the "main" quintesson Regis Cordic
But my all time favorite is Victor Caroli as The narrator!

I think we've covered all the nominees.
I'll add Skywarp. He's easily the hippest Decepticon persona, even if Rumble tries to be.
Soundwave is the winner though. He & Optimus are the quintessential Transformers no matter how many hundreds more are created.
I think it's because he comes across so robotic yeah? We immediately bought into all of Soundwave's traits early on because he was the most 'transforming robot' of them all.

I'm glad to see Astrotrain. He doesn't get nearly enough love for how cool his voice is.
I also put Lord Zarak in the top coolest voices.
Can't stop the signal.

The Combaticons did have cool voices, especially Milton James as Blast Off and S. Marc Jordan as Onslaught. Dead End had a cool voice, but we didn't hear much of it.

This won't be popular, but I love Rodimus Prime's voice in Season 3. He had a great tone with his snark and then sounded nice and commanding when needed to (and not whining about being the leader).
Optimus Prime's original voice (and not movie trailer guy like he is in the movies).
Soundwave
Megatron
Silverbolt
Hound
All the Combaticons
Cyclonus (Roger Carmel's version)
Jazz
Omega Supreme
Blaster
"They have a grill, it's this grill. Now you have itit's called America."

This is a tough one since Transformers had such a great cast. I'd have to say my favorites are
Optimus Prime
Unicron(movie)
Galvatron(movie)
Soundwave
Omega Supreme
Springer
Jazz
Cyclonus
Ultra Magnus(movie)

1st and 2nd season: Soundwave and Jazz. The Insecticons are pretty cool as well.
3rd: Hmm.. Blurr in some ways, and like some of you, Springer. Just thought Springer was awesome overall.
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Unicron and Galvatron (Movie)
Springer (Movie/3rd season)
Soundwave
Cyclonus

Soundwave hands down!!

Soundwave, Shockwave & Cyclonus for different reasons. Soundwave had the coolest 'robot' voice, Shockwave had that detached, clinical sound to his voice and Cyclonus had the cool, calculating voice that so many Decepticons lacked. Other Decepticon voices I always liked were Starscream, Megatron, Galvatron (tv version), Astrotrain & of course, Rumble's Brooklyn 'tough guy'.

Soundwave and Shockwave, hands down!
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SOUNDWAVE.
Jokerz77, you get brownie points for mentioning Starscream; my favorite Transformer.

Use the Force.

Meh. Starscream was too "screechy and whiny" for me. lol
And Unicron of course has the best voice to me in the overall Transformers universe, but I was looking more at just the TV series and not including the movie. lol
Also, yes Optimus had an awesome voice, but it was just "normal" sounding in my opinion. Strong and John Wayne like? Yes. But too "human" to have that awesome robotic quality to it. lol
Oh, and one more thingyou just read my signature.

Blaster and Starscream had cool voices.

Optimus, Unicron, & Soundwave