I'd say that the Hulk's iconic look is The Rampaging Green Monster In Ripped Up Britches, and anything deviating from that is less iconic...
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...but it tends to be less of an issue for Hulk. Those changes in look for him are usually story powered, and so the fans tend not to lose their marbles over it.
Ditko gave Dr. Strange a very definitive look, and he tends to always return to it. However, he's had several alternative looks without it being that much of an issue. Interesting to note, though, that his most iconic look was not the original. That came along after quite a long journey of development, culminating with The Ancient One giving him the Cloak of Levitation and Eye of Aggamatto. Before that, his look had been a bit more sedate. For his least iconic, I'd say it's his 1990s look where the cloak is illustrated more like a duster...
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...you can still recognize him as Dr. Strange, but it's a long way from his iconic, vaguely East Asian-inspired look.
This may be a little more hair-splitting than you had in mind: Silver Age Green Lantern had a specific look for decades, and it was the design for the entire GL Corps (except for the occasional GL with a completely non-humanoid physiology). I think it was after Guy Gardner broke the mold with this biker-vest look that we began to see character-specific variations on the costume, and it's been in some degree of flux ever since. If I have to pick one of those, it's probably the minimalist look they gave John Stewart during the JL toon era...
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...of course, DC has since foisted so many GL designs on us, you might be able to argue that there no longer is any iconic GL look.