Most Iconic Score for a CBM.....

[Bayne]

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.

 

[The Caped Knight]

Easily Alan Silvestri theme The Avengers!

 

[Brother Jack]

Silvestri has kind of been the unsung hero of the MCU. Endgame and Infinity War are the best superhero film scores in years.

 

[B]

The most iconic is still John William's Superman, 40 years old & it's still the standard by which all others are judged, Danny Elfman's Batman would come in not far behind.

As great as Alan Silvestri's Avengers is, I don't think the Williams/Elfman, main themes at least, are ever something that can be eclipsed as being more iconic.

 

[STARMAN]

The thing that impresses me about Williams' score is the versatility of the entire soundtrack. It isn't just the main theme. It's also Planet Krypton, Leaving Home, Fortress of Solitude, Can You Read My Mind? The soundtrack has science fiction, fantasy, romance, whimsical, heartfelt, and triumphant. It is the variety that appeals to me.

 

[B]

The thing that impresses me about Williams' score is the versatility of the entire soundtrack. It isn't just the main theme. It's also Planet Krypton, Leaving Home, Fortress of Solitude, Can You Read My Mind? The soundtrack has science fiction, fantasy, romance, whimsical, heartfelt, and triumphant. It is the variety that appeals to me.

The entire soundtrack is great, obviously the main one that everybody knows & recalls is the Main Theme, but as you mentioned there are a few other hidden gems that are perhaps 'lesser known' but if you played them to anyone that had seen the film they'd recall them.

The Lex/Otis theme is one of the more underrated tracks & it's actually a little tune I randomly whistle from time to time -



When I hear that I always thing that is such a perfect little melody/tune for the way Lex/Otis were portrayed in the film.

 

[The Endless]

Williams's Superman. Elfman's Batman and Spider-Man. Then Silvestri's Avengers.

Those are THE iconic superhero themes for me. Nothing else comes close. Superman and Batman are a few levels above Spider-Man and Avengers though.