I go the other way. I love 'em both, and have 'em both on DVD, but I’ve only seen OUATITW once. The harmonica music really drove me kinda nuts, though the first 10 minutes of it are drop-dead gorgeous.
I watch TGTBATU every couple years, and it never gets old. And it has the single best Western score ever.
I am with @daneel here do I have to pick?
Both are amazing films that need to be seen on the big screen.
Once Upon A Time is a more traditional western revenge story with good guys and bad guys and a much more American feel to it. It was filmed out in Monument Valley instead of Spain. Henry Fonda pulls off the bad guy role really well. Sergio Leone had a big budget for this one and that shows through in the sets, stars and production values.
The Good, The Bad is more stylish and a heist film with no real good guys. I like heist films. I like good looking movies that pull it off on a non hollywood budget.
They are both great westerns yes but still different types films so it is hard for me to pick.
I really enjoyed that movie. Great action sequences and crazy plot and just a whole lot of fun and the directors love for the genre really shines through.
What about casting against type, as they did with Fonda in Once Upon a Time in the West? Most actors these days seem to embrace a wider gamut of roles but there’s gotta be a contemporary actor* that’s up to now been cast usually as a neutral or good character.
*If your answer is Tom Hanks, hit yourself. Hard.
ETA: Actually, why stop at re-making it as a Western? Set the remake in Africa and cast Elba. I don’t care what role, just get him to sign a binding contract. Throw in John Hurt and take my money.
ETAA: Okay, now that I’ve given this some thought:
Idris Elba as The Good:
…Adepero Oduye as The Bad (bandana reveal ~1 hour in):
…and John Hurt as The Ugly—not because he’s ugly (quite the opposite) but because he’s John Hurt: