As I'm a player, I don't know all of them, only the ones we've been offered. Twice we were offered the ability to move about Avernus magically concealed, which was really important given the fact we're pissing everyone off. Both times we needed to slay a powerful fiend, and each time we considered the probability of needing to slay this fiend anyway. Both times we declined, but we really wanted to (the last one we didn't do because it would take us too far out of our way).Exactly! Does that book offer any good suggestions? (I've only ever skimmed my copy because it reeks of glue from the binding and I end up with a headache if I try to read more than a page or two at a time.)
There was a "fiend" of Tiamat (I don't know if he's actually a fiend) who's offered my character two deals, both of which I've taken. The first was on the Prime, he was trapped in a summoning circle while we fought a bunch of devils. I was in the back dealing with lesser devils, but the rest of my party was getting slaughtered. I made a deal with him that I'd free him in exchange for helping us defeat the devils (plus depart without harming any of us), and he saved our bacon. The second time we negotiated for thousands of days of food to be delivered to a city in need of help in exchange for destroying a rival devil warlord that we were going to pass anyway.
That same "fiend" offered a particularly nice deal to one of the PCs, as he'd already show great disdain for making deals with fiends. It could only be taken by him (I tried), as it was an attempt to disrupt his "pure" ideals. There's a celestial being trapped somewhere on Avernus, and he'd tell us where if we slew a band of demon worshipers that were traveling the River Styx. The PC refused, because he's been trying to shenanigan his way into finding the celestial without making the bargain (we've gotten a basic idea, but not enough to act on).