Has to be beholder with mindflayer as a fairly close second.
Weird lizard freaks like kobolds are in so much non-d&d fantasy stuff that they shouldn't be mascots.
If you showed anybody who didn't play (or even someone who did) a picture of a red dragon they wouldn't necesserily think d&d, therefore they shouldn't be the mascot.
Owlbears are just some other mutant freak like a minotuar, centuar and many other monsters.
Now beholders and mindflayers are unique to d&d they don't look like they belong to some other fantasy setting.
If I showed them to some one who didn't play d&d they'd be like, "wtf is that thing?" Not, "oh look a <insert generic monster here>!"
And they aren't half something, half something else (interesting but not terribly original these days, since half somethings have been done for thousands and thousands of years).
And I just think beholders are plain out cooler than mindflayers. Therefore I go with beholder.