In a vacuum I think it’s Ariel. I’ve found her mana buff + Telluria’s mana impairment to be extremely annoying. But if you’re already flanking Telluria with Vela then you probably don’t want another blue flank. If your blue spot is already spoken for then I think I’d favor Kunchen.
I’m disinclined to Guinevere due to the conflicting HoT. I don’t think the mana synergy is enough, given that there are AI and timing issues. Rigard is great, but I don’t think I’d opt for him over a five star hero. So of the group you listed it’s Kunchen for me, kind of by default. That is, I’d prefer to just put in snipers or AoE attackers.
Normally I wouldn’t flank Kunchen because of his mana speed, but Telluria makes her flanks so likely to fire that I think I’d be okay with it. Deep heal and he produces an ailment, which is really what you want to slap the other team with. Telluria really challenges them to keep up and cleanse constantly, so just pack the field with ailment producers.
Worse defense I’m probably seeing so far is something like
Drake, Black Knight, Telluria, Vela, Gravemaker
Gravemaker, Freya, Telluria, Vela, Costumed Joon / Poseidon