Judging only on-pitch behaviour:
Suarez before he went to Barcelona. When he went there, he seemed to mellow out and hasn't done anything awful that I know of; but before then, he was by far the most despicable player I'd ever seen. Non-stop diving, cheating, intentionally attempting to injure players, biting, deliberate handballing, racism... just pure filth on the pitch. This cnut bit another player three different times. It seemed pretty clear that he was literally mentally ill. While there have been other players who were just as bad with the diving or thuggery, noone else that I've seen has been such an all-encompassing degenerate. The way he would drive his studs into other players' ankles and then feign injury himself just made my blood boil. He was just completely bereft of any sort of sportsmanship or common decency, to a point that far exceeds what people might excuse with silly notions like "he grew up playing in the streets in a poor country." Countless other players did that without turning into despicable swine. I was pretty much convinced that he was legitimately a sociopath.
Then one day he just stopped doing it. I suspect his Barcelona contract included a very sternly worded clause. That or he went into therapy or something.
For sheer unlikeability, Neymar is a close second. He's not as psychotic as Suarez was at Ajax and Liverpool, but he's showing no signs of maturing and will probably continue to be a petulant, cheating manchild for the rest of his career. It's just that he doesn't go around biting and punching people. I also couldn't stand the hysterical, shrieking victim complex that all of Brazil had when he was injured at the world cup, but I suppose that's not technically Neymar's own doing.