I have always thought that the first tour was the best. It might be a case of "freshest is best." I think its exactly because they didn't really know it that it was allowed its own head. Its spirit came out the strongest as a result. It was more freewheeling, more organic on the first tour. The performance that I like the best is the one on Wide Awake. It has a swing or a groove to it that later versions completely lack.
I think that the song became such a showpiece that it kind of affected the performance of it. Have you ever done something brilliantly the first time and then every other time after that you try to recapture that performance but you can't because of self-imposed expectations? I imagine with a song as naturally loose-limbed as Bad is, that the minute they got into the mental frame that Bad had to be great, a big centerpiece with a big payoff, that they sucked some of the life out of it. They would have to. In order to make it a consistent highlight, they had to put some discipline in it. Bad plus too much discipline is, well, need I say it? Let's just say it was not a good thing.
I may be wrong but it seems like Bad kinda faded away (that last pun was so not intentional, BTW) in the 90's. It was semi-retired there for awhile. I think it did it some good because when they played it regularly again it was as close to new as it could be. It started to fly & groove again. That to me was always one of the best things about it live. It was such a wonderful treat to hear it like that, so free, with something extra not found on the album. As opposed to the fragility of the album version, it was a wonderful character change to the full and strong sound of the live performance, almost like having two completely different songs from the one brilliant idea.
Sorry for the length of this reply but it just got me going on one of my favorite U2 subjects, that is anything and everything Bad. (How many songs after all can be a whole subject anyway. Not freakin many I'll just hazard) Funny thing is that I was just listening to it on UF and the thought crossed my mind to dust off my Wide Awake and listen to that version too. Then I saw this thread. Now I just gotta go and pop it in.