The 'definitive' themes, for me, means both iconic and perfectly representative of the characters. There are a lot of themes (Elfman's Spider-man, the X-Men, arguably GotG) which are nice pieces of music that tell you absolutely nothing about the characters (or in the case of Elfman's Spider-man, tell you absolute nonsense about the character). As for iconic, I'd agree that Williams' Superman and Elfman's Batman are the only unquestionably iconic themes available at the moment, although I think Zimmer's Wonder Woman and Silvestri's Avengers are far enough down that road to start calling them iconic, as well. Also, I strongly suspect Goransson's Black Panther to be following shortly, although it's too soon to be certain of that just yet. Scores like Man of Steel, Captain America and Ant-man I think are incredible pieces that perfectly represent their characters, but will almost certainly never be iconic because their time is over already (or in the case of Ant-man, because the character will just never be popular enough to be that widely known). And then there are a few dark horses which, in my opinion have what it takes to be definitive if they can become iconic and may still have the potential to become iconic. I'm thinking there specifically of Dr. Strange, who isn't really widespread enough yet, but his appearances in other films seem to be driving his popularity significantly higher than I'd ever expected, so his excellent theme may still have a shot. And also of Giacchino's Spider-man which I think is a lot of fun and a much better representation of the character than Elfman's, and the way they've sort of fused it together with the old cartoon theme - while unexpected - is certainly memorable.