I have PP1, PP2, and Spy Hunter, all in a row. Personally, Spy hunter is my favorite, followed closesly by PP1, then the test track on PP2.
As far as other people playing, they gravitate toward the PP2, and test track. Its by far the easiest of the group.
You can read my sig for a list of the other games that i have in the PP1 and 2 cabs - of them all, the easiest to play is the final lap 2's - and they're linked to each other, which is a fun experience for 2 people. Outrun is ok, but it loses a bit using the PP wheel without any kind of feedback. Top Speed is basicly an outrun clone, but plays much better in the same cab. I'd be hard pressed to get rid of any of the 3 i have, but, if i had to just keep one, it'd be spy hunter, as it was the first game i ever got the high score on.
Its been mentioned before, and i so want one as well - race drivin or hard drivin either. Unfortunately, its a medium res, so i really can't add it to my current setup without an auto switching tri-sync monitor, which is a bit more than i'm willing to spend. Plus, i think they're much better in the sit down version. Maybe someday i'll have the space for one. But, if i were just going to buy one, and i had the room, i'd go for a race or hard drivin sit down. Its still one of the best imho.