Think I may go for at least one Ludmille. Dual cast TM is a long term winner which will always be useful.
I'd caution everyone to not go hard for Lightning. She's awesome, and will be MVP for a long time if you get her, but if you already have 1 or 2 dps units that get a 6* upgrade, you should be fine. Cloud, Bartz, and Exdeath have 6* forms right around the corner (Cecil too, but he's shit dps). Chizuru, Garland, and Firion get 6* forms a few weeks after that, and everyone gets 6* Rain and eventually Lasswell too.
As awesome as Lightning is right now, she's overshadowed by even more OP units down the road.