While Apoc is really good, Doom is just next level.
I'd say rank up either Doom or Abs Man. Doom is so good that you'll get bored using him. He kills pretty much everything.
I somehow missed Doom in your list, even though he was first. I change my original response to Doom. He's my most used champion (even over my r5 Herc). He's also great for Act 8 too.
Doom or Apoc, can't go wrong with either. For solo play Apoc just tackles soooo many different nodes/champs that can be annoying. His disorient immunity was so valuable when kabam was spamming those disorient nodes anywhere and everywhere they could.
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I'd say rank up either Doom or Abs Man.
Doom is so good that you'll get bored using him. He kills pretty much everything.