Simple: either the one that ie most relevant for the type of content you play, or the one you enjoy playing the most.
This. Choose who you like the most and enjoy playing. I almost considered r4ing my onslaught despite the fact I rarely play him and don’t even have him in my BGs deck simply because of his prestige. And then I snapped out of it. Legit prolly taking Hawkeye up instead just because he’s more fun and this game is bleeding itself dry of fun, so I’m making my own. Personally Nefaria is a blast and incredibly durable.
Yeah I'm currently leaning towards nefaria the most, especially since I want to try doing stuff like necropolis etc (haven't touched any of it yet). I feel like if I went with bastion or onslaught for bg's they'd just end up getting banned every single game as this is only my 2nd r4 and since they don't see that much use in quests they'd just be sitting there.