Very difficult choice here, if there was an "All" option I'd go for that!
Breaking it down in minor parts:
For pre-Group B cars there isn't really any competition for the Lancia Stratos, it's great looking, won almost everything, and is basically a Ferrari gone rallying
For Group B cars I feel like saying the Lancia S4 because it so well represents that era and the fearless drivers needed to tame these cars.
For Group A I'll once again go Lancia, Delta this time, again because it did cut pretty well. A close second would be the ST185 Celica, simply because of the packaging that contains it
World Rally Cars is where things get really tough to choose, so once again splitting it into minor parts:
For 90's WRC I think I'll go with Impreza '99 or either of the Lancer, as these cars remain iconic to this day. Worthy mentions are Peugeot 206 and Citroën Xsara, for the success that was to come for these. In regards to Ford Focus, the real 1999 or 2000 liveries would make me appreciate it even more.
For post-2000 WRC it's no doubt the 2003 Impreza, mainly because it's as of right now the last non-french car to win the WRC Drivers Title, and partly because I've meet the driver in person. The 2003 Lancer Evolution however is also a very good car.
Rally Raid is also no real biggie, Pajero '85, because Paris-Dakar winner.
For what I call "Rally Modified" cars, it's pretty easy: Impreza '05 RM if you don't have the DLC, Evolution X if you do. I like the looks of both and they're pretty equal in performance.
I don't have anything to say about the premium '08 WRC cars, I've said it before and will say it again, except the ST205 Celica, they've chosen the wrong one to premiumise (here's hoping we'll have McRae's '95 Impreza along with the Lancia Delta we don't know about)