I'll just cover the Evo 6 range, rather than all of them since time began......
Evo 6 RS
Stripped out version, designed for Group N rally preparation. No air-con, ABS, AYC. Also does without Recaro seats and Brembo brakes. Comes with steel wheels, quick ratio steering rack (2.2 turns lock to lock) and short ratio gearbox. Brakes, seats and wheels can be added back into spec. as factory fit options. Only available in Scotia White (I think). Japanese only model, but available through Ralliart UK for rally prep. purposes.
Evo 6 RS Sprint
Ralliart UK version of RS, with Stage 1 engine upgrade. Boosted power to about 330bhp.
Evo 6 RS-2
Based on RS version, but has air-con, Recaro seats, 17 OZ alloys and Brembo brakes. Also has longer ratio gearbox from GSR. Available in Scotia White and Silver. Japanese only model.
Evo 6 RSX
Ralliart UK version of the RS-2 model. Same specification.
Evo 6 GSR
As RS-2, but with ABS, AYC (active yaw control), climate control. Has different steering rack (2.7 turns lock to lock). Much wider colour choice available.
Evo 6 RS Tommi Makinen Edition
As RS, but with modified suspension setup (better suited to UK roads). Also has re-styled front bumper that loses holes for fog/driving lights and Tommi Makinen monogrammed Recaro seats. Dash instrumentation is also changed with Red on black lettering. Available only in Scotia White. Japanese only model.
Evo 6 GSR Tommi Makinen Edition
As GSR, but with modified suspension setup (better suited to UK roads). Also has re-styled front bumper that loses holes for fog/driving lights and Tommi Makinen monogrammed Recaro seats. Dash instrumentation is also changed with Red on black lettering. Similar colour to choice to GSR. Unique colour (Red) with rally decals also available.
I think that's about it. If I've made any mistakes than I am sure someone will correct me. Those models I've marked as Japan only have never officially been imported under the Mitsubishi UK banner, but are available through grey importers.
Personally, I own an RSX, and think it's brilliant. If you are thinking about getting an Evo, then any of the above would be a good choice. If I'd had the money, then I would probably have gone for the standard GSR.