Kaye (obviously) Burton and... I hate to say it, but Weaving. And I only pick Weaving because when Megatron wasn't roaring or shouting post-TF07, he had this really awesome voice that rode the line of soothing and sinister. When he returns to the Fallen in ROTF and talks about the Allspark, I really love that performance just a bit more than Welker's in TLK. But only by a bit. Welker in TLK was probably the best he's ever been. The slight dip in pitch and the boom it had accented his already great performance into something. Shame it was wasted.
Had to go listen to all the ones I missed. Frank Welker, Hugo Weaving and the unmentioned Gary Chalk.
Yeah modern Welker is a different animal entirely than G1 Welker. Though I admit he actually did sound depraved and a bit intimidating using his G1 voice for the 2007 DS game. Kaye and Burton are both so smooth and intimidating. I also enjoyed Weaving even if Weaving himself wasn't invested in it. I also thought Tatasciore was fun as a loony wannabe revolutionary in WFC and FOC. Honorable mention to @Chris McFeely for his take on IDW Megs!
You should change the question to say voice actors not just voice. There's a difference. Frank Welker for example does not portray G1, Prime, and movie Megatron's in the same way. Despite being voiced by the same person they have three different voices. G1 Megatron was more raspy than the others and to be honest I've never really liked this Megatron because it just sounds like trying to talk with larangytis. Though in a way I like it because it's the only one of the three I can actually sound like... at least without straining myself. The deeper sounding tone of Movie Megs I could do but that hurts my throat so I don't like doing that too much too often. Prime Megatron oddly is some where in the middle like a nice balance between the other two extremes. And oddly that balance always seems the hardest to get to, for me at least. David Kaye also has two different Megatron voices but in his case it's technically not that he's doing a different voice, although UT Megatron does over enounciate his "yes" the way Beast Megatron is famous for. But the thing that really sets them apart is that Beast Era Megs is just David Kaye's voice. UT Megs was put through a robotic synthesiser as were all the voices in the Unicron Trilogy. This tends to make everyone sound distorted as if they're talking into tin cans. The end result of which means even though they're both the same actor they end up not having the same voice. Though in this case you can still easily tell even with the tin effect that they're both David Kaye. That effect barely makes a difference but it's still worth noting. Anyway my three favorite Megatron voices are Animated, Prime, and Beast. As explained above I don't like G1, and while it barely makes a difference I really can't stand synthesizers. I can tolerate Sound wave, just barely, but when it's as poorly edited as the Unicron Trilogy was it just makes it worse.
His performance in Prime was better but he had better written scripts to go with it. I loved him in the early episode with the psychic cortical patch. I thought his performance was fantastic in that episode. That said, I did slightly prefer the metallic enhancement to his voice in G1.
