I'd say my favorite would have to be Kid Tripp, but it is only because i defined retro here in a very specific way. Would old classics ported to iOS count as retro? Would games like Chrono Trigger, the earlier FFs, or even the later (but still pretty old) like Little Big Adventure, or KOTOR count? Or you just mean pixel heavy with basic gameplay mechanics even they have been created in a newer setup like VVVVVV or Adventure to Fate series?
Greetings, I would include both ported classics (retro proper) and contemporary games (retro styled) in the listing. And you can include however many you want really but it is good to establish some general guidelines - 20 is a nice big number.