I figure I can only give an opinion on what I know. I have learned a little bit about top fuel dragsters lately, however I know little to nothing about F1 cars, though I can say that I'm sure they are impressive in their own respect (I might look into learning more about them sometime). Rally and road race cars have always interested me I just don't know enough about them to form an opinion on them either.
Since it seems others are straying from the fields listed in the OP though, I gotta say, I am most impressed with cars that are built by your average joe. Ones that are true street cars, but can still perform at the track (I'm generalizing when I say track here) too. While these cars I'm talking about are not professional race cars (in most cases), they impress me more than a purpose built car that was built by a team. I think I am most impressed by them because of the amount of passion and dedication and thought that go into them coming from an individual, the fact that they are a labor of love. Something you probably don't see too much of in professional racing.
TL;DR: I am impressed by all forms of cars and racing but its the people behind it all that impress me the most, especially the average joe who builds his own car.