In terms of music. Options are:
1988-1993: When he sounded like an LL Cool J rip off
1996: When he sounded like a Nas/AZ rip off
1997 - 2001: Batshit crazy, Slim Shady, off the wall dont give a fuck lyrics.
2002: More emotional tracks
2003: Kind of the start of the gangster phase he went through, but still had some dope verses.
2004-2008: When he sounded like a pilled up white 50 Cent
2009: Serial killer lyrics with accent
2010 - 2012: His current style.
Personally I like '97 - '01 the most, just because thats when Em was pretty much at his peak lyrically, and that's also when he was stirring up all sorts of shit with the gay community and what have you with the MMLP.