There can be only one - well, not really. Everyone has their own preference of where to cap spell levels at.
The preferred cap for 38% of the poll responses was 6 magic-user / 5 cleric, which corresponds to OD&D.
Right behind that was 9 magic-user / 7 cleric, which of course matches up to AD&D. That came in at 33%, but by pure numbers only a difference of 7 votes.
Capping at level 9 for magic-users and clerics is a 3x concept if I'm not mistaken, and came in at 24%.
Honestly, it's a bit across the board, and not one answer was overwhelming in it's responses.
Kinda tells me a have a nice cross section of mostly OSR type readers on this blog, but also a chunk of 3x / Pathfinder / why play a game with levels in it readers too.
That's okay, in the near future I should have my copies of Drinking Quest in hand. I suspect that game will appeal to readers and gamers of all editions and rule sets ;)
(there were 127 total responses to the poll - thanks to all who took the time to respond)