Really it depends on two things, how convincing the champion is and how good his challengers are.
Ric Flair is the classic example of a successful heel champion, spending probably 75% of the 1980's as a detestable bad guy while being champ almost all decade, drawng crowds, TV ratings, etc. Flair however was an exremely entertaining character in his prime, outlandish interviews, and he also delivered in the ring against almost everyone. Still, Flair was at his best when he had a true legitimate "ggod guy" to oppose him, someone fans could really believe was capable of being champ for a long time. Kerry Von Erich, Dusty Rhodes, Magnum TA, Lex Luger, and Sting all fit this mold, and Flair drew good money in showcase matches against all of them. Fans not only have to dislike you as a heel champion, they have to legitimately believe that your opponent can beat you AND remain champ.
Likewise, Flair was entertaining and his matches were good but fans were not interested in him as a bad guy being chased by Kendall Whyndam, Brad Armstrong, and only moderately enthused about his fued with the Garvins (though the NWA put a lot of props in hat fued such as Jimmy putting a date with his wife vs Flair's title and Flair's insistance he would retire if he didn't beat Ronnie at Starrcade).
Same hold true for fan favorite champions. Hulk Hogan became stale and much less interesting once he had vanquished Randy Savage (twice), Roddy Piper, Ted DiBiase, and Harley Race. Putting over the Ultmate Warrior and returning to "save the title" from evil Sgt Slaughter was fun, but after awhile the parade of overweight monsters and muscle heads with no wrestling skills became dull, as did Hogan, once all the legit title threats had been vanquished.
Brett Hart was a great wrestler but his time as champion draws mixed reviews because of the perception he didn't draw well. However, with WWE in a state of flux during those early years, having lost most of the bankable superstars from the 80's and struggling to replace them (kinda like now) how many top guys did Hart face that you truly believed could beat him. Hart certainly had a lot to do with the rise of HBK into main event status, but if he didn't perform s well as some would have liked at the box office how much of that was due to lousy booking and a lack of quality opponents.
Same thing happened in WCW with Sting. Nervous that he would be overshawdowed by Flair, they immediately ended their program after STing won the title, even turning Flair good for awhile in a fued with Ron Simmons and Butch Reed, but WCW had few bankable villains to match against him. Fans loved Sting, he sold lots of merchandise, but no one bought Barry Whyndam or Sid as guys who were good enough to beat him. For whatever reason they didn't let him battle Luger, one guy big enough and accepted enough with fans to be believable as champ. Eventually they put the belt back on Flair, but Sting's lackluster reign was less a reflection of his talent or work ethic than it was WCW's failure to find him/build him up believable contenders.
Personally, I enjoy seeing the good guy chase the title, because if he wins its a great fan moment, title change, good triumphs over evil, seeing the hated villain vanquished. This is why almost every WrestleMania ends with a fan favorite champion winning. The only time I enjoy seeing a "babyface" champ chased by a "heel" is when the heel is someone like Flair or HHH, or even a bad guy Undertaker, someone who has the gravitas to be champ, that you know has the skills, but also a character that you know could believebaly win the title straight out talent wise but is cunning enough to swerve it away. I enjoyed Edge pursuing the title for this reason in recent years, just as I enjoyed watching Flair chase Rhodes and Steamboat years earlier.