I have heard almost every Beck song ever over the last fifteen years, so it's difficult for me to recommend only one.
I'll run down the albums and hits for you:
Mellowgold - Loser, the big one. I love this song. I love most of the songs on this album, I broke-up with my first love while one of the random tracks was on so it has a special place in my heart. Think garage-rock production with hip-hop influences.
Odelay! - The ultimate classic. Wall-to-wall excellent songs all produced by The Dust Brothers, multiple hits - Devil's Haircut, Where It's At (the 'two turntables and a microphone' song), New Pollution; collage style hip-hop and alternative-rock that sounds like everything and nothing else all at once. You have to hear it at least once while you're alive, it's simply one of the great albums of the 90s.
One Foot in the Grave - an indie release, hard to find but one of my favs, all folk and country-rock songs with some abstract and minimalist flairs about it. An interesting, introspective listen.
Stereopathetic Soulmanure - I blame this album for making me the absolute weirdo bastard I am today (and I love it). A random mishmash of outtakes, live cuts, bedroom demos and answering-machine messages that defies all attempts at categorization. Unlike Beck's other albums, it approaches all array of styles and genres without ever tying them together in any sort of cohesive fashion. Only for the brave and dedicated, also rather hard to find.
Mutations - a soft-rock album more akin to today's 'indie' bands, I believe it was produced by Nigel Godrich who did the best Radiohead albums. Wraps up a number of softer styles in its ethereal textures for a completely different experience from the previous albums. It was when this album came out that we knew Beck was going to be a fresh treat with every release.
Midnite Vultures - It was when this album came out that we knew Beck was fucking crazy. A 180 degree turn from the preconceptions set-up by Mutations, it's an album-long tribute to 80's R&B and proto-hip-hop cheesiness. Sexx Laws is the first track and it's James-Brown-via-Rick-James vibe is the only way I could properly prepare you for what this album has in store. You'll either love it or hate it.
Sea Change - Beck got really depressed and made an emo album. Not really, because he isn't any more whiney than usual, he just hit his Mutations stride in a really really poor mood (after ending a 7+ year relationship) and turned out some of the most hauntingly beautiful music I've ever listened to. My favorite is Lonesome Tears, the strings (contributed by his birth-father, an Oscar winning film composer) are heart wrenching. Nigel Godrich also produced this album, and his ethereal touches make the album the great headphone experience that it is.
Guerro - A good but not memorable album, E-Pro and Girl are it's best tracks. I think of it as Odelay-lite. Benefits from production by The Dust Brothers.
The Information - If Guerro was Odelay-lite, this album is Guerro-lite (or maybe Crystal Odelay...) Did not stick with me at all, but I do recall it having some good tracks.
Modern Guilt - By this point I feel that Beck has lost his creative spark that made his younger work so great. It's a feather-weight listen that doesn't benefit from Modest Mouse's piecemeal production.
So I'm really surprised to hear Beck had a track in NBA 09, I'll have to check it out for sure! I've been really hopeful that Beck will get his spirits back in order now that he's doing his "record a famous album in only a day" project.