I love them all !!!! I guess since there are thousands of vent frees in my area, I'd have to say the Peterson G10......I know they don't have a big fan club here, but someone's got to keep them properly maintained.
....now the list of ones I'd prefer not to work on is growing !!!
I work on all brands of fireplaces, we don't have ventfree in our area (Ontario Canada) but I would have to say the newer ones with the ability to pull the entire burner tray out to work on the valvetrains are my favs.