The encounter's type of AoE is largely going to determine who is best between Bard, Summoner, and BLM. If they're clumped up and low HP, the BLM is going to win out. If they're spread out so that the Bard's extra range is hitting more things, obviously he wins. If the adds are going to survive a while, namely long enough for the BLM to need to go into regeneration mode, AND the Summoner can prepare a little bit (dropping Shadow Flare, getting up close to pop Miasma II, already having the 3 spreadable DoT's ticking, etc.) then the SMN wins.

Honestly as long as the SMN can prepare to AoE and Bane will actually spread the love to everything (IE they need to be damned close) there's nothing that's going to stack up to that. W/ just an Ifrit book my 3 DoT's + Flare do 400 damage every 3s and I'm freely able to nuke and do other things during this time. It is just EXTREMELY rare that the stars all align in any given scenario for a SMN to beat a BLM. In general though, BLM > SMN > BRD > lolEverythingElse for most scenarios.