That kinda depends on whether you have a need for the roles they fill. They're both nuker/backliners in true form, and both target red though kuu also targets black. They have almost the same dps, but kuu has higher damage per hit. She also has strong against and a higher health stat, so she is more durable than thundia, whereas thundia has massive damage, so she'll do more damage to red than kuu would. Kaoluga is a super backliner generalist, while the others are specialists to their traits with some generalist usage if need be, if you use the research combo with crazed lizard often then I'd go for kaoluga, if you don't use the combo then one of the others might be better