Rimming is probably my favourite sex act (especially giving it), and there's something about rimming a top in particular which is a huge turn on for me. It's something to do with the way a lot of tops (not versatile guys who are topping me, but guys who identify as "top only") consider their ass so off-limits, so when I get a chance to go there it never fails to make me rock hard.
There have been 2 occasions which turned out to be the most memorable:
The first was a guy who told me he had tried bottoming but didn't like it, so he only wanted to top. I asked him ahead of time if he would let me rim him and he wasn't too keen on the idea at first, but I guess I wore him down because he eventually agreed to it. I get the feeling he only said yes because of how much I told him it turns me on, I don't think he was particularly looking forward to it. Because of his bad experiences bottoming (and because I've received my share of bad rimjobs from guys who had no idea what they were doing) I started off with a really light touch, using the very tip of my tongue around the edges of his hole before gently licking the outside of his hole without trying to push my tongue inside like I normally would. I did this for about 5 minutes before I decided to wrap it up, thinking he'd prefer to move on to other stuff, but as I moved to get up he said "uh, it's ok if you want to keep going". I ended up rimming him for about 20 minutes, and towards the end he eventually got used to the sensation and relaxed enough that he let me work my tongue inside him. I haven't seen him for a while but I like to think I helped undo the bad bottom experiences he had earlier.
The second guy I met on Grindr - he was a 19yo uni student and told me he wasn't interested in anal, he only wanted me to suck his dick. When he came over I made a move to kiss him and he backed away, explaining he had only just started fooling around with guys recently (I was only the second guy he had met to hook up) and he wasn't into kissing yet. I figured this would be a standard blow and go but he was cute so it was cool with me - I pointed him towards the bedroom where he took his pants off, kept his shirt on, and sat on the edge of the bed. I kept all my clothes on, get between his knees and got started (he had a really nice dick, a little under 5 inches long but a nice medium girth and perfectly proportioned). After a little while he put the heels of his feet on the bed frame so his knees were raised and leaned back onto some pillows. I took the chance to taste his balls and started rubbing my thumbs on his perineum and along the sides of his arse crack. I started licking underneath his balls and looked up at him - he was looking right at me with a kind of half-worried-half-hopeful look on his face. I asked him if it was ok for me to keep going and he gave me a slightly unsure nod. Like the first guy I knew he would prefer a lighter touch, so I didn't try any penetration but lightly dragged my tongue around the edges of his hole before licking his rim. I remember he had a really "distinct" hole - the wrinkles around the edge were really prominent, and his actual sphincter stuck out a bit too, just like a mini donut. I had never rimmed a guy with a hole like this before but it was amazingly sexy. I could feel the tension in his muscles (both his cheeks, which I had in my hands, and the muscles of his hole under my tongue) but eventually he was able to relax and I could tell he was enjoying it. After a while I went back to the blowjob and he came really quickly. After I finished swallowing him I stayed between his legs, and he kind of stretched them out and lay them on my shoulders and back for a bit while he rested. Eventually he lifted one leg back up but kept his other one over me, which opened up his crotch again and I started rubbing my face in his balls before I lifted them out of the way and started licking his hole again. I could hear him saying "yeah, yeah" under his breath but he didn't get hard again (there was something really "cute" about his dick when it was soft) so after about 5 minutes I got up and he left. He messaged me on Grindr later and confirmed that was his first rimjob, and he liked it better than he expected, and although I tried to arrange another hook up I didn't see him again.