Realizing this is completely unscientific, below is a list of car services that have generally received overall positive comments on this forum. Comments, suggestions, recent experiences?
Note: Ray's London Transfers has recently been mentioned by a couple of people because they were unable to get any confirmation of the reservation, however, one poster said that Ray has updated his computer system so as to avoid these problems.
Note: I have removed Simply Airports from the list as recently there seems to be a significant number of negative comments about them.
From my own experience, we used Exclusive Airports last year and were happy with their service.