I got this idea from the "Thor" thread, where people have been comparing Jane and Pepper Potts. It seems to be a requirement that every superhero movie these days has a woman in it who the hero is interested in romantically. This character doesn't always feel organic to the plot or like a fully rounded three-dimensional character. She is sometimes reduced to a damsel in distress or a plot device to motivate the hero.
I'm surprised that Peggy Carter wasn't brought up in the Jane and Pepper debate. She's my pick for the best Marvel love interest so far. She is one of the most strong, independent, and well-developed female characters in those movies. She doesn't seem to exist just to moon over and/or flirt with the hero. She has a lot of ideas and motivations that have nothing to do with the hero and doesn't seem as forced towards being romantically involved with him as some other superhero love interests.
How she and Captain America come to care for each other feels very natural and earned to me and by the end of the movie I felt deeply for both of them. Their chaste and ultimately doomed romance provides my choice for the most poignant relationship yet in a Marvel superhero movie. It's a big part of why I loved Captain America's first movie.
While the relationship between Spider-Man and Mary Jane is certainly more fleshed out, I thought it was a little belaboured at times and too soap opera-ish, although it did help make the end of "Spider-Man 2" very powerful (before the sequel killed its resonance).
With polls like this, there are inevitably some choices people might like to see that get left out. There may also be choices tht people find questionable. I tried to be as thorough as possible, but people will probably be disappointed by some exclusion or even inclusion. I'll address the few I can imagine people bringing up.
In the area of possibly questionable inclusions - I haven't seen "Ghost Rider", but I assume that the character played by Eva Mendes is the love interest. Elektra is more than a love interest as she's also a hero and even has her own movie, but in her case, I think it's possible to be both a hero and a love interest.
I haven't seen the Punisher movies, so I don't know if there are love interests in them. I saw "Blade", but I don't remember if there's an important girl in it. I didn't include Rogue, Jean Grey, or the women of "Fantastic Four" in the poll because I wanted this to be more about solo superheroes and their ladies (and I didn't see those movies anyway).