Associate
I think Gaius might be the worst boss on reflection, it just felt wonky all the way through with absurd damage spikes and dodgy attacks.Agreed DLC is incredible. Bosses generally felt fair...I haven't done bayle or Gaius but that's more down to personal preference. I think my favourite was Putrescent Knight.
Bayle was probably one of the best just for the pure spectacle and the all in NPC that you can summon, the camera was still iffy at times though, but that's the case for most of the large scale enemies/bosses.