Rain is a very shit character (he's free after all), but he does go to 6* and has fullbreak, so there's that. Lots of alternatives though.
Ling is the latest and greatest breaker, but she was a limited time unit so if you don't have her, she's gone. Noctis can handle breaks, and do extreme damage at the same time, and that's what most endgame teams use now.
I think that's about it for 6* breakers? I may be forgetting someone, so anyone feel free to chime in. Warrior of Light becomes the king of breaks for a long time once his 6* releases (AoE 45% breaks with 3 turn duration).
Mages can be good if you load them up with TM's. For example, Dark Fina, Exdeath, or Emperor in 100% best in slot TM gear have around 800-950 magic. If you them give them a Ring of Lucii for dualcasting Alterna, that's a 1530% AoE magic nuke, which is going to be completely absurd damage per round.
Problem with the above is mana. That mage will run dry by round 3, and you could use a physical dps in less TM gear, to do almost as much damage that can go 6-8+ rounds before needing to worry about refilling their mana. Physical hits generally chain better too.