Saint Cross would pretty much be my suggested "must have" after FB and MB.
Thief's Revenge suffers to needing good thief weapons in synergy realms to make the skill shine in U/U+ alonside Shout and other debuffs.
Full Charge is somewhat similar, as while it can individually break 20k, it's something SC can come close to without needing more action time and more easily if holy weakness is in play.
Power Chain may cut FC's use time, but you also gotta look at it as combined use time. For perhaps 1.6 moves, you get 25k damage or so in harder content. Meanwhile, looking back to SC again, 2 rounds could net you 40k. Frankly, I find PC more useful to trigger SBs quickly.
Dark Zone kinda comes off as the mage version of SC, has the MM synergy, and can now be employed in most realms for the sake of CMs. I can't speak of the chain spells since I don't play JP, but this would be my choice after SC if coming up fresh.
I can't really bring myself to say Bahamut is worth it with how summons level. It's a lot of orbs for just 3 uses and you need an incredibly pimped summoner to really make it shine over the MM/DZ combo. I've only ever used it in the summoner dungeon, if that's saying anything for viability.
Other stuff like Doppleblade, Poison Grenade, Flashing Blade, and the dragoon abilities are more a matter of convenience for daily/orb farming. Yeah, they've had some occasional boss handiness, but if you chance upon a decent SB for someone who can equip Life Siphon, odds are you're just going to be spamming that with them, anyway. For example, Gabranth is technically the strongest character I have now with his armor if a mob is dark weak, as 70k in a single use is possible. Meanwhile, Lightning with her BSB is probably closer to 40-45k due to how low the multi-hits are mathed out and there being no element to exploit.
XI Wiki



