So, there's a heuristic that I think is a decent one, which is that less-conscious things have less potential suffering. I feel that if you had a suffer-o-meter and strapped it to the heads of paramecia, ants, centipedes, birds, mice, and people, they'd probably rank in approximately that order. I have some uncertainty in there, and I could be swayed to a different belief with evidence or an angle I had failed to consider, but I have a hard time imagining what those might be.

I think I buy into the notion that most-conscious doesn't strictly mean most-suffering, though - if there were a slightly less conscious, but much more anxious branch of humanoids out there, I think they'd almost certainly be capable of more suffering than humans.