I'd say Cuban's the best owner in the NBA and it's not even close, although I could see how he could rub people the wrong way.
Worst is somewhere between Clay Bennett + company, Ted Leonsis and the guy that owns the Magic, esp. after Sterling's formally given the boot.
Clay Bennett?
Last time I checked, the Thunder are in the conference finals and Celtic fans are pining for a 10-year draft-reliant rebuild that's never going to work.

And the reason Bennett is bad is what, exactly?
Pure ownership:
Scuttled a legitimate contender under the flimsy excuse of not wanting to pay the Luxury Tax, then got the league to reimburse the team for Kevin Durant's contract extension (
http://www.sbnation.com/nba/2013/7/18/4536924/oklahoma-city-thunder-kevin-durant-contract-nba), Refuses to fire Scott Brooks and replace him with a competent coach.
Unpleasant basketball maneuvers:
Conspired to steal the Sonics from Seattle. Possibly by ordering the GM to divest the team of talent in order to keep attendance down while making absurd claims on a publicly financed arena, although the GM part is speculation on my part.
Unpleasantness elsewhere:
Since I was saying "Clay Bennett + co," I'll point out that Aubrey McClendon owns Chesapeake Energy Corporation, which makes its money off of fracking, something I'm not down with.