Like several others, I've never sucked my best mate's cock (nor he mine) but we have wanked each other off a few times.
The first time, when we were at college, we were crashed out in his room following a late night out drinking and we had been talking about wanking and other sexual things. Both of us had partners, mine working at the other end of the country, his abroad. We were both very horny and quickly got hard. After a while he just announced that he had got to have a wank, unzipped his jeans and pulled out his cock. I followed suit, given that he was going for it. I was fascinated that his uncut cock stood fully vertical when erect, hard against his stomach, the head nearly touching his navel. I can't remember now, I made some comment about it and asked about how hard it was. He let me wrap my hand around it and attempt to prise it off his stomach. It was so hard that it would barely move. He asked to feel mine, so I let him. Then we started wanking and at one point grabbed each other and stroked each other's cock.
That became a regular occurrence for a while, following the first time, then less frequent. We wanked in his room, we wanked in my room, we wanked in the living area, we even wanked in the kitchen a couple of times, late at night.
We never progressed beyond a bit of mutual wanking though (and we're still mates to this day).