From purely a bgs standpoint, Bullseye and Onslaught need the r3 more to further lower the chance of getting nuked. Despite you having them both as r6a, I'd still pick either one of them. Can't really speak much about werewolf, he's cool but I can definitely speak against venom and I will till the end of time.
Unless you have a high rank 5-6* relic, deep wounds maxed and -preferrably- the dupe, venom will probably suck in bgs. Even then he doesn't answer many meta defenders but well, he can take nick fast I guess so there's that. He is so much fun with the animations/simplicity though and is the most enjoyable out of the bunch imo. Just not as relevant as the others in bgs.
TLDR: Pick Onslaught or Bullseye for bgs. Venom for simple fun. I'd personally pick Onslaught myself.
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Unless you have a high rank 5-6* relic, deep wounds maxed and -preferrably- the dupe, venom will probably suck in bgs. Even then he doesn't answer many meta defenders but well, he can take nick fast I guess so there's that. He is so much fun with the animations/simplicity though and is the most enjoyable out of the bunch imo. Just not as relevant as the others in bgs.
TLDR: Pick Onslaught or Bullseye for bgs. Venom for simple fun. I'd personally pick Onslaught myself.