I would want to say Raiko but I haven't seen and heard the actual scene yet, only the animations; so it wouldn't be fair.
From what I already have seen I'd say it's an equal spread for the most part.. There's only one that really sticks out of the crowd.
That being Gabriel.. both of her scenes. Her voice and moans just tickle my senses; it's invigorating.

If you were to fully hear her lines out, she definitely has the longest scenes that I know of running at ~9:30 and ~11:30min.