For Gigan, I say 1972 version. The '73 suit was just bad, and while I like the 2004 design, I don't think it works as a 'definitive' design. It's just too radical.
Actually, I think "EX Gigan" would fit better in an Ultra film/show than in a G film (like a few other things in GFW).
Ghidorah - King Ghidorah, specifically - is a lot harder.
Showa Ghidorah probably holds up the best still, but there are multiple small details about this suit (measly-looking manes, stubby feet, lack of great size, etc.) that I think really need improvement. Unfortunately, they're really subtle details, and Toho seems to have difficulty with them....
Heisei Ghidorah succeeds at looking a little more fearsome, but that's about it. At first glance, his size appears to have improved, but at second glance you notice that this Ghidorah is very lanky - there's not much mass to him. I don't like his flexi-horns either.
Grand King Ghidorah seems to have the opposite issues. Although he is definitely the most fearsome-looking Ghidorah of all, he is now lacking in another aspect - he barely has any horns. Ghidorah needs a mane, be it of horns or hair, and GKG only has the main "crown" horns. Another issue is that GKG looks too massive, and his wings appear a little too small.
GMK Ghidorah, I admit, is growing on me; It's actually a pretty good suit. The body mass is almost perfect, and the varan-esque horns work surprisingly well. My favorite aspect is that this Ghidorah has TALONS, not those pitiful stubby toes he's always suffered from. All the issues with this suit (short necks, small wings, facial design, etc.) are due more to the role he filled, not the suit itself. Nevertheless, these issues mean that this design can't work as the "Definitive" King Ghidorah.