A lot of atmas suck, so don't think you have to get all eleventy million of them.
Here's some of the better atmas; what BST should use will vary by situation, though. Refer to something like the Atma
page on wiki.
^ RR and MM up there are pretty much game-breaking atma and redefined Abyssea for both melee and mage respectively once Scars of Abyssea came out. Definitely get these ASAP if either melee or mage roles are important to you.
Misc (some of which are useful for BST in particular):
Apocalypse (grants unlimited Reraise)
Ducal Guard (the original game-breaking atma for pet jobs; reduces damage taken by -50% when HP is below 50%, and combines with other gear/traits that reduces damage taken for pets. Exact workings of this atma have been tweaked here and there and are unknown to me, but as far as I know this is still a commonly used atma by BST and SMN)
Zenith (unconditional, but weaker, reduction to damage taken)
Tremors (same as above)
Earth Wyrm (same as above)
Future Fabulous (provides a high Magic Defense Bonus, great against the right NMs)
Cloak and Dagger (evasion boost, helps Dipper Yuly tank)
Siren Shadow (same as above)
Sand Emperor (same as above)
Mounted Champion (20/tic Regen; strongest unconditional Regen atma. Also boosts VIT and reduces enmity loss)
Stronghold (15/tic Regen, also moderately boosts attack and defense)
Impenetrable (15/tic Regen, also reduces breath damage taken)
Dread (Treasure Hunter +1. In general though, this might not be worth sacrificing another atma for, unless you are with a bigger group that has no problem with killing things)
I'm sure there's more, but I need to wrap this post up quick. Might edit more in later. Edited, Aug 24th 2011 6:17pm by Fynlar