JBL isn't the worst WWE/F Champion ever, but he's not that great either. He isn't bad in the ring, but he's not World Champion caliber. Workrate might not be everything, but I believe the World Champion should be at a certain level, talent wise, and JBL just isn't there. His mic work is pretty good for a midcard heel, but, again, it isn't World Champion level. Same with his charisma. And his current halo gimmick might be amusing, and it is to a degree, but you shouldn't be laughing at the World Champion like that. You can't take a character like JBL seriously when he's booked like he is, and that is fine for the midcard, but it's all wrong for your World Champion.
The worst WWE/F Champion, at least as far as one who was meant to be the main guy to build things around, is easily Diesel. Had to be carried to anything above average, not that good on the mic, and the worst drawing champion ever.