I personally think that Relapse, and Infinite are his best two albums.
Infinite because he is just fucking sick. The songs 'Infinite, Maxine, Never 2 Far, Jealousy Woes, Backstabber, Open Mic, It's OK' I can't even fuckin pick which one is better. No beef. Just some good ol' rhyming.
Now Relapse... There is so much shit. I'm mostly talking about the unreleased things. I'm having a relapse, Things Get Worse, Syllables (sounds older though) and all that fun stuff. Then records on the album like Same Song and Dance (<--- My personal favorite), Music box, Stay wide awake. My top 3 on the album. They are just amazing, and everyone is like "SLIM SHADY!" Slim Shady wasn't released until the fucking Relapse album.