I am unsure what is the "theme song" of many movies. For instance for Robocop, in the beginning it has almost no music at all, but what I consider most memorable music in it is what plays during the end credits (and bits of that in certain parts of the actual movie).. Or, <span class="bold">the music for which I remember Blade Hunter</span>, isn't it heard only during the end credits too, nowhere during the actual movie?

Also, I very much liked much of the music in Alexander (the movie about Alexander the Great), but I have no idea if any of them is considered as the "theme music". For instance <span class="bold">the music during the battle of Gaugamela</span>?

Anyways, since Robocop was already mentioned and I try to mention some a bit (but not much) more obscure:

1. Revenge of the Ninja (not sure which I'd consider the "theme music", they can both be heard during the end credits, in this order):
https://www.youtube.com/watch?v=5ehR4p8JCb8#t=40
https://www.youtube.com/watch?v=rih_XC8iqZo

2. The Underneath
https://www.youtube.com/watch?v=FYTJTgO_WTY

3. Powder
https://www.youtube.com/watch?v=jehWTbyuQHk

Hate? Ummm... Maybe Titanic. My heart will not go on. I guess i just heard the title music far too many times even before seeing the movie, like the rest of the world. Sometimes too much is just... too much.

A lot of awesome pieces so far thank you. I'm definitely going to add to my playlist the themes of Akira, Strange Days, Dredd, Star Trek (so so good) and Powder. I already have Last of the Mohicans and Blade Runner's themes in my playlist.
I would add Midnight Cowboy's theme as well, but I can't really listen to it. It reminds me too much of the film, and I'm still not over that ending :P

a glorious slip up XD

But yes, I don't think it's always possible to unanimously identify a specific piece of music, if any, as the theme of a film. Sometimes the most iconic piece of music for me is played (again) during the end credits, eg. this incredible piece from Independence Day or the Blade Runner piece you linked, and sometimes it is played during the opening titles, eg.Stargate.

The Shining
Young Frankenstein

Can't think of a third...

Despised the theme to The Omen.

Predictable;
Rocky
A Fistful of Dollars
The Magnificent Seven

Worst
Robin Hood Prince of Thieves

Awww sheet.... Blade Hunter just sounds better to my ear. "Blade Runner"... if you are running on blades, won't you get cuts to your feet? Or is it some ice hockey movie?

I'll add another top three- three great underrated (or just not often mentioned when it comes "the best") themes.

Mask of the Phantasm

Star Trek Generations (I know many people hate the movie, but I like it a lot, and I absolutely love the soundtrack)

Robin Hood Prince of Thieves I'm going to cut your heart out with a spoon!

Ah yes westerns, some truly incredible themes there. Fistful of Dollars is a favorite of mine, and off the top of my head so is
High Plains Drifter, and [url=https://www.youtube.com/watch?v=i3Q8h-fDfEI]Once Upon a Time in the West. And can't mention the latter without the hauntingly powerful Man with Harmonica .

Of course the best western theme is , or perhaps rather [url=https://www.youtube.com/watch?v=AOHbrsklpWU]this version of it.

Uhm, but as for:

I'm afraid I'm going to side with Breja on this one :) The Robin Hood Prince of Thieves theme has long been in my playlist.

We've got a guy here who ran on blades and was called the blade runner, Oscar Pistorius. I'm kind of annoyed that the press decided to call him that though because 1) Blade Runner is my favorite sci-fi of all time and 2), because he is now a convicted murderer :P

Oh yes, Morricone's music for all of Leone's westers were superb. I also really like the theme for The Quick and the Dead, just like the movie itself obviously an homage to those Morricone/Leone films.

Two things, I clicked expecting a Batman theme and was initially incredibly confused to hear a cheerful and eerily familiar tune (the Time Warner bit). Also, once the choir kicked in it immediately struck me as incredibly familiar and I couldn't continue listening until I could figure out what it reminded me of.
Then it hit me, the music at the culmination of Outcast's story : Heaven on Adelpha. Specifically compare the part at around 0:50 in Mask of the Phatasm to
the part at around 01:00. Though of course preferably listen to the entire piece because its an epic tune to fit an epic game :)
Morricone is definitely one of my favorite composers. It was his music for The Mission that made me look him up for the first time. I guess I should also actually watch the film sometime instead of just listening to the music over and over :P

And I love The Quick and the Dead theme as well. It's one of my favorites in my 6 Disc compilation of nothing but themes from Westerns. Sadly the music outmatched the film for me. Not that it's a bad film by any measure though.

Favourites:
Alien Theme - Jerry Goldsmith
This will always be #1 for me.

Princess Mononoke Main Theme.
Nausicaa of the Valley of the Wind Main Theme.
Both by the great Joe Hisaishi (Mamoru Fujisawa).

Hate:
Ghostbusters

One I was also thinking of listing was the theme from The Edge:

https://www.youtube.com/watch?v=K8qL30R9RPk

but I figured one Jerry Goldsmith work is enough for one list (ie. Powder). Jerry had quite a similar style in many of its works that I used to mix up the themes of e.g. Powder and The Edge, not being sure which theme was in which movie. Anyway, I still like them both.