I think probably three of my guy (all three straight cis guys thus far) friends. The first two were two of my best friends at a week-long music festival a few years ago where we were camped out and partying. We were drunk and I decided it was time to tell them that I'd come out as trans and they were both like "yeah, that's cool makes sense," and one of them mused about what it would be like to be a woman, lol. Then they spent the rest of the festival advertising me as their brother to everyone in the general vicinity. It was pretty awesome.
The other was an online friend I hadn't talked to in yyeearrss. I recently bumped into him online and told him what had been up. Because of some of his previous beliefs I thought he might freak a bit, but he was just like "oh, that's totally cool, no worries" and went on talking about totally unrelated politics.
Those were probably my best experiences.
That and one of my profs in uni a while back. When she first called my name for an assignment I got up to get it and she was like "oh, I thought that was a guy's name" and I slinked back to my seat kind of embarrassed and not wanting to say anything in front of the whole class. Then she was like "oh sorry, maybe you're a guy" and I just sat there embarrassed and trying to disappear into my seat some more. But afterwards she was quite cool about it and I didn't even have to come out to her. She just used male pronouns the rest of the semester and ended up being quite nice...that was pretty awesome