I've been thinking about this a lot lately and I'm more conflicted than ever.
It's pretty much a tie for me of:
-G2 Gamera. As others have said, he's the perfect mix of Hesei Gameras. There's nothing overtly goofy about the design and he's not so scary that he can no longer be considered a children's character. This is also, I believe, the best designed Gamera of the bunch. It's just a solid creature design, even if you take away it's Gamera.
-GOTU Gamera. A perfect update of the Showa Gamera. A very appealing design for kids and it seems very Japanese in concept. When I think of an ideal image of what Gamera should look like this is usually the one that pops up in my mind. There are some angles of this Gamera that look amazing, and some that look straight goofy as hell...but that's exactly what Gamera should be.
-Showa. Specifically, the 68 version I think. Might be nostalgia but I love the Showa Gamera films and the suits.
These three are tied for me and I like them all for different reasons.
If I had to pick one it might be the G2 suit...or the GOTU...or Showa....it all depends on what day you ask me