I think "best" is a bit subjective here. If you want one zeta to do a whole bunch of things, zFinn is my vote for "best." (haat, GW, tb, lots of pve events)
But if you are trying to max out your arena team in a competitive shard, it's actually pretty noticeable. Remember, he inflicts 2 debuffs on both basic and special. Any attack (including counters and assists!) boost your team a little.
IBAT isn't going to save a garbage team. But it WILL help you win otherwise evenly matched fights. In that regard, winning more pvp and dropping less overnight could classify IBAT as the "best" option.. Depending on your situation.
As a bonus, it makes p1 haat solos a bit easier. So that's nice too I guess.