Actually, a few are still deleted, such as yours telling that non-member to either join or stop posting xD
I think Dragonite, Charizard and Salamence will be the best to ride.
As seen in PokeSpe, an older Aerodactyl is stand-on-able if it's big enough, but Red must get painful shoulders being dragged around like that...
Dragonite's fairly bulky, so there's plenty of space to stand on. Salamence is kinda flat and rounded in its' central body, and Pokémon Hunter J clearly demonstrates how to stand on it and look cool/evil :P
Charizard is quite big too, and you can ride on it, again like Green/Blue in PokeSpe does.
Flygon would be possible I guess, although a bit small....the same goes for Altaria.
If by "flying" you're referencing their typing, then Gyarados would be pretty good for riding...it's massive, and you can either grab onto the horns like Red, or stand just behind them holding a Plate looking menacing like Silver...
Sorry if you don't understand this, all you non-PokeSpe-readers.....but it's how my brain works xD