Buchannan was a solid fighter, fundamentally sound and very gritty. However he didn't punch very hard and as Floyd would say, he didn't have any "special effects". Mayweather would win a one sided decision if he didn't stop Buchannan on cuts.
His most famous fight was probably against Roberto Duran. It was a close fight but finished after Duran hit Buchanan with a low blow which stopped Buchanan from continuing and the ref awarded it to Duran. There was meant to be a rematch but he never got it.
On the fight I saw of him the other day his eye was messed up bad pretty early in the fight and he still went on and got the win, he almost stopped the guy in the 14th but he made it the distance.