Tina is good for PVE content to bust tanks. Sieg is ok, but requires you to build a mana team around him generally. Abelle is pretty unnecessary for content clearing - more of an annoyance in PVP. I hear good things about Kyle, and am currently trying to skill up a Kali based on some of Vaelek's videos.
So, TDLR - Tina for energy adventure team, Sieg / Kyle for mana team.
My recommendation for content clearing is to get Piet's debuff skill maxed and also build a giant bat. Using a team of Hekate, Giant Bat, Piet, and Jules I have been able to clear 4/5 towers.