I tried out about 20 rally cars testing and tuning at grand canyon and the Lancia Delta S4 walked away with the title. I haven't really tried any normal cars. Also everything was stock power (except I upped the '99 Subaru to 400hp - it still wasn't too fast). Here is the list of my times for gc, I didn't drive perfect but they are pretty clean laps, clean enough to not incur 5 second penalties if they were in races. Also I try to minimize the banging of the rear end against barriers, walls etc.
Some of the cars only got a run one way, some got both forward and reverse times. The Lancer Super Rally Car got a little poor showing because it was one of the first to go and I got better later - but this might give you an idea how some different rally cars performed under similar circumastances.
Normal
1: Lancia Delta S4 Rally Car '85 - 2'28.342
2: Subaru Impreza Rally Car '03 - 2'28.944
3: Ford Focus Rally Car '99 - 2'29.056
4: Peugeot 205 Turbo 16 Rally Car '99 - 2'30.263
5: Subaru Impreza Rally Car '99 - 2'31.444
6: Mitsubishi Lancer Evolution Super Rally Car '03 - 2'31.458
7: Mitsubishi Lancer Evolution IV Rally Car '97 - 2'34.504
Reverse
1: Mitsubishi Lancer Evolution Super Rally Car '03 - 2'29.943
2: Subaru Impreza Rally Car '03 - 2'30.564
3: Subaru Impreza Rally Car '01 - 2'30.637
4: Ford Escort Rally Car '98 - 2'31.179
5: Peugeot 206 Rally Car '99 - 2'31.554
6: Subaru Impreza Rally Car '99 - 2'33.280
7: Mitsubishi CZ-3 Tarmac Rally Car '99 - 2'33.647
8: Lancia Delta HF Integrale Rally Car '92 - 2'35.678
9: Toyota Celica Rally Car (ST185) '95 - 2'38.272
Alot of the cars I tested got bumped off 'cause I would forget to clear the board and have 4 Ford Focus times push off my Celica and Stratos etc. I wish the rankings list was infinitely long. I will probably have to resort to writing times down so I won't lose track of cars that don't make the top 7 or 8
Some cars I couldn't get to compete despite looking promising at first glance:
Mitsubishi Straion - no better than about a 2'38.xxx
RS200 Rally Car '85 - around 2'35 or so
Toyota Celica Rally Car (ST205) was the same as (ST185) roughly but got bumped off.