I'd say Aldo.
I almost dismissed him cheating against Mendes, but then there was the (alleged) piss story and him getting amateurly starched against the "joker" McGregor after failing to show up the first time. The follow up didn't help, as he complained about UFC payment and made a campaign to (rightfully) fight Conor, only to back down when they (supposedly) gave him the opportunity, even if on short notice.
The way he seemed content at essentially being gifted the belt after Conor left was kinda embarrassing, and though nowadays I don't fault him so much for that, some of his subsequent performances - especially his losses - left me feeling he wasn't as good as he was hailed to be in terms of heart and skills. Of course he had some mileage by then, so in a way it's understandable.
In general, fighters that I like but that barely improve piss me off - Condit and Barboza come to mind -, but I guess the fact they don't complain much keeps me from rooting against them.
EDIT: Shevchenko also comes to mind. Ducking and whining/giving excuses aren't doing her any favours.