I suck at making lists of things, especially "favourites" or any sort, so I'm not sure I can even make it to five. With that in mind, in no particular order (well.....apart from the top two being the ones I mainly want to see):
<------- I've always liked Ebirah, mainly because I've always just thought that lobsters are cool, kinda weird-looking creatures that actually exist in the real world, so the idea of a giant monster one has always appealed to me (that and Ebirah: Horror of the Deep is one of my favourite Showa films).
And yes I know he's technically a giant shrimp, but he's still a lobster to me, dammit!
King Ghidorah. Save him for the third movie though (if this were to be a trilogy), really build him up and do him right. I've love to see a huge knock-down, drag-out fight between Godzilla and KG with American blockbuster special effects and proper respect for the characters. Have the second movie be a big, crazy Destroy All Monsters-type affair, then bring it back to just Godzilla and King Ghidorah for the last one, and really play up how screwed humanity is now that he's arrived on Earth, even more so than when Godzilla first appeared (and use him to complete Godzilla's turn from the "villain" of the piece to a 'still not a good guy, but at least kind of an anti-hero who's sometimes the lesser of two evils and can actually help us out now and then when another giant monster shows up').
Rodan. Because Rodan. He's a classic, and I've always loved his "roar". I wants to hear that in one of these movies.

I could go with Gigan as well. He'd be a cool one to see an "Americansed" version of. Keep the alien cyborg origin, because again I've always wanted to see an updated version of his flying crystal intro done either from the point of view of a TV news camera man or perhaps just someone filming the crystal on a camcorder or phone, then crash-zooming in when it explodes and he first appears in the sky, a la the original films.
Megalon would be cool too, though they'd probably have to change him into just being a giant cockroach-type creature and leave out the drill arms. I'd have no problem with that. I always thought it'd be cool to see him appear by burrowing underground through a city, then rising up out of the ground
inside a tall building, which explodes from the ground up as his rises through it floor by floor, then for it to fall away and have him walk out through the smoke and debris.
Biollante would be another good one, but that's six now and yeah as I said I suck at making lists. So whatever.
