Doing a little bit of browsing on Marmit's site gives you a good overview of the kits that have been released, or re-released as is often the case.
It only goes back to the early/middle part of 2006 and it looks like the only Toho kit I can't find pictures of is what I think is the very first one, the 1964 Godzilla. Most of the Iron Joe kits have been re-released at least once and I'm almost positive the 1962 and 1973 Godzilla kits were also very early (2004/05) releases because of how crude they both are:
Arranging the Marmit Toho kits by order deadline shows that as of right now almost everything has been released twice so far. Keep in mind not all of these releases were put out as built-ups.
Hedora - 10/15/06
Gezora - 11/15/06
Ganime - 11/15/06
Kameba - 11/15/06
Mogera - 12/15/06
Gorosaurus - 1/15/07
Megalon - 2/15/07
Gigan - 2/15/07 (though this one ended up being delayed almost a year)
Angilas - 3/15/07
Godzilla 1962 - 3/15/07
Godzilla 1973 - 3/15/07
Megalon - 11/30/07
Angilas - 12/31/07
Heodra - 1/31/08
Gorosaurus - 2/29/08
Gezora - 3/31/08
Kameba - 5/31/08
Ganime - 7/31/08
Assuming I haven't lost anyone by now it's obvious the Gigan (I'm pretty sure 2007 was it's first release) will probably be offered again. The kit I'm most curious about is the Mogera:
Beautiful isn't it? I'm not sure how many times it's been released but I hope I can get another shot at it in built-up form. Looks like one of those be-all, end-all pieces.