My favorite design would be G1, but I mean as he is portrayed in the movie. The actual action figure leaves a lot to be desired. It's a great time to be the biggest Galvatron fan right now as I can choose from at least three different 3P versions of my favorite character. Or not choose at all and try to eventually own all three.
Me I love the colors they used for the car robot/rid Galvatron upgrade the white and purple was fantastic I also liked beast wars to Galvatron And of course G1
If I were to pick one design it would be the original G1 designs in the cartoon and comics not counting his toy bio appearance. Purple looks great, and I love the very prominent points on his crown. I know Galvatron II has different colors, but he looks great, too. I also really like IDW's design, although I don't like it as much the original. AOE's design is underrated. His face is very visually expressive, he has an underrated Voyager toy, and the vortex grinder in his chest does look badass.
Legends Galvatron. A Megatron who's a headmaster to a Galvatron which has both a homage cannon and a sci-fi tec jet mode. Just awesome. Energon Galvatron would probably be next.
The TFP design, which was technically Megatron in a new Dark Energon armor and possessed by Unicron but I'd call it an unofficial Galvatron design.
G1, BW2, Energon, RID, SG, IDW Alltough i dont really consider the RID and UT designs Galvatron designs cause apart from a new colorscheme and RID gaining a few extra modes they arent any different from the megatron design. I really like how energon gave him a flightmode.
Even though I appreciate each of his designs over the years, (some more than others) If I had to pick a favorite It'd probably have to be his G1 / IDW designs.
IDW Galvatron is my favorite Galvatron design. I just love how spiky and powerful he looks. Runners up would be IDW Galvatron's Earth mode and TR Galvatron.
G1 Galvatron. I just think the crown-like helmet and huge silver and orange cannon coupled with that cannon mode was a killer upgrade to Megatron in G1. I'm a bit more partial to the Marvel/toy colors, though, as they help to make him feel more like a reincarnated Megatron, and give him a different color scheme than Cyclonus.
I've only seen the G1 and AoE versions of Gavaltron and surprisingly I prefer the G1 version much more despite being introduced to the AoE version first. This is one of the only Transformers that I feel this way about.