After watching it again a couple of days ago, my nomination goes to Clint Eastwood’s The Unforgiven. Probably half of the movie is rain-soaked.
Other nominees?
Seven
Hard Rain
The Perfect Storm
“In Search of Noah’s Ark”
Blade runner. I don’t think it ever stops raining.
Purple Rain
My Darling Clementine
“Rain” (1932), probably Joan Crawford’s best performance. “Doesn’t this rain ever STOP?”
Good rainy musical numbers:
• “Pettin’ in the Park” (Gold Diggers of 1933)
• “Run, Little Raindrop, Run” (Some Betty Grable/John Payne movie)
• “I Love to Walk in the Rain” (Shirley Temple/Bill Robinson)
• “Singin’ in the Rain” (either the Ukulele Ike or Gene Kelly versions)
I knew I could count on Eve.
But come on now; there have to be some more good rain-drenched film noir entries out there.
Some solid nominations so far. (I was wondering if anyone would cite My Darling Clementine and Bladerunner.)
Though the movie as a whole is not rainy, there is a well-done downpour sequence in Angel Heart.
Just caught it on DVD, lot’s o’ rain…
Castaway
All Summer in a Day.
Pretty much rains from beginning to end in that one.
Rain Man?
:: Ducks and runs away, fast! ::
::in shame::
Highlander II
Gawd, Munch, you saw that too? involuntary shudder I got four free passes when it was in theaters. After we walked out my three friends beat me senseless in retribution.
But there’s another reeeeeal bad rainy movie not yet mentioned:
Godzilla
Matthew Broderick spent the entire flick looking like a drowned rat. Still haven’t figgered out why the director made it rain the whole time. Musta been to cloak the animatronics and second-rate CGI.
The Crow. Heck, that’s even a key song (and significant line): It can’t rain all the time.
Munch and Tygr,
You have caused me great pain by making me remember having seen THAT movie… It hurts…oh God, it hurts… make it go away Mommie…
I have to second the BladeRunner vote. The next to last scene with Rutger Hauer and Harrison Ford in the rain was phenomenal!
Forrest Gump. “You call this a storm?!?” 
the Venus segment of “The Martian Chronicles” first comes to mind, as the rain is not just the setting, but the antagonist.
Jurassic Park, The Lost World: Jurassic Park, and, judging from the trailers, Jurassic Park III.
The Poseidon Adventure
Sorcerer, driving a truck across a rope bridge in a monsoon.