I don't have a favorite slot machine, either - but then again, I hardly ever play slots these days. I usually only play slots when I'm out with friends for a relaxing night out and then I don't really care which machine I play on.
I think that there have been a lot of new releases since I last played slots, but when I did play last my favorite machine was Top Dollar. I think it's my favorite because I won money on it.
For me it's Airplane. I think you're right in that we mostly choose our favorite based on how much we have won from that particular machine in the past.