Brown by a country mile. Had he not suffered through the injuries from 99 through to around 01, I'm not 100% sure we'd have even signed Rio. Fergie rated Brown as the best young defender in England when he broke through. Injuries curtailed what could have been an outstanding career with us and England.
Evans comes next, way more reliable than either of the other two. He could read the game well and defend better than the other two, the only thing that potentially pulls him close to Smalling and Jones is that he isn't quite as versatile as the other two. I still think we sold him 2 or 3 seasons too early.
Jones is third on my list, ability wise he's above Smalling and perhaps had things panned out as they could have done, would have been Brown's long term replacement. However, as with Brown he's lost so much of his career to injuries, that have robbed him of what potentially was a very very good career. Like Brown at the same point in his career, he needs a couple of games to get himself playing well for the first team, the problem is that he's still picking up silly niggling injuries, for which he can't be blamed totally for. He just doesn't seem to get a decent run in the team to prove himself anymore.
Smalling, plucked from non-league by Fulham and it shows. Far too often he has brain farts and makes stupid errors. Under LVG he thought he was some sort of ball playing defender, but all he'd do would be run to the centre line and then pass the ball straight back to the opposition. Over came a spell where like Jones he couldn't stay fit, but even in his best season with us under LVG when we won the cup, a lot of that was down to Blind who told him where he needed to be and who to mark, rather than his own ability. He nearly cost us the cup final with his stupidity, he was in such a poor position that he had no option but to get himself sent off. So glad he's away from the club and hopefully never comes back, we've got way better coming through and could easily buy better players than him.