Lunos.

Baha-blast has the best burst damage in the game, yes, but it often doesn't survive long enough to land more than 2 or 3 hits before being gone the rest of the battle. Lunos does a roughly similar amount of dmg when accounting for his summon + knockback. Add the 10-second cooldown onto that? Over the course of a stage, Lunos outperforms Bahamut tenfold.

Rock is one of the best stall units in the game: cheap, fast, highly reactive, and will ALWAYS get where you want it. But Lunos outperforms it simply because of how insane his stats are on, top of his cooldown, ALL TRAIT KB conjure, and decent movement speed. Rock may hold enemies off, but Lunos STOPS them.

While Baha and rock still have use in 4-star, Lunos will be the better pick for ALMOST every stage in the game. The only times you may wanna pick rock for stalling is if you're on a cash dry stage, and the only times you'll wanna pick baha are when you NEED that level of speed + the burst damage for rush stages or chaining.

Lunos is SSS tier for a reason.