Eminem is a whiteboy who can rap. His whole mainstream career is a gimmick. The 1999-2002 period was amusing and he put out great quality music during the time. After 8 Mile he started taking himself seriously, trying too hard and his releases took a big decline (The Eminem Show is so corny when you look at it for what it is). Of course, at that point Marshall Mathers was heavy on drugs and had no creative control whatsoever while Eminem was the brand that had to be exploited to the maximum. I believe the record execs pushed him far beyond the mark he was aiming for and he couldn't handle it. Eventually he ran out of gas and fell off the map for a couple of years.
Relapse, for what it is, is about as honest as Eminem can get and it was not even his intention. The themes speak for themselves - drugs and the insanity that comes with abusing them. What is great about the album is the atmosphere it creates, save for a few tracks. The serial killer theme and the accent might put you off at first, not to mention the stories of Eminem being fukked by his stepfather but it's all a part of the mess that's going on inside his head. Everything you do catches up eventually. The production is flawless - Dr. Dre is a producer that can make a timeless classic if you pin him down onto your project and cut him off from the rest of the world. Eminem is a godly writer if he gets down to it. This was the perfect case - they were both out of touch with the world, Relapse sounds like it was recorded in timelessness. Apart from some songs that were obviously forced into the album for some mainstream appeal, this was an album that had nothing to do with the rap game at the time.
This is a good thing because Eminem's next album Recovery was the exact opposite - template singles, missing subject matter, production all over the place, basically all the marketing tools were there and the artist's personality was completely missing again. As a result, Recovery is unlistenable today and Relapse is a piece of art.
Relapse is not a compilation of tracks designed to make record sales like every Eminem album after it (I forgot how many they were but it doesn't really matter). I'd rank it at #3 behind MMLP and SSLP, maybe on a good day it can even be #2 because MMLP has a few serious flaws.