Right, now to answer on Shearer...
IMO he was undoubtedly world class. He never played in a dominant team. Even when he won the league with Blackburn, he was playing in quite a limited team mainly of workhorses. His goal scoring consistency in the teams he played in was fantastic, and as has been already pointed out here he performed pretty well in competitive fixtures for England when pressure was on, again in a non-dominating team.
However, thinking about Shearer does highlight how the game has evolved. He was regarded at the time as the 'complete' player - he had strength, an incredible shot, and was great in the air. If you watch youtube highlights of him it's surprising how many goals he scored with his head, especially at Blackburn so many of them were from crosses from the byline from wingers, you don't see those very often any more. Jason Wilcox and Stuart Ripley used to bomb it down the wing and deliver balls in to the box and Shearer would time his run beautifully to head them in.
Today, Shearer would not be regarded as a complete player, he wasn't that quick, he worked hard for the team but always with a sense of him being the end product, he wasn't a great link up man and he wasn't in it for the assists. I think Shearer was regarded as the undisputed Premier League best striker until Henry came along and started bagging loads of assists as well. It would be interesting to see how he would be used in the modern game. You could use him in a one up front no problem as he was strong enough and could hold the ball, but I bet he wouldn't like all this business of laying it back to attacking midfielders or inside forwards, when Shearer got the ball usually he ended up having a shot.
I think the way he played probably suited Blackburn and Newcastle, he was the main man and it made sense to play through him and he would score a shed load of goals. When he had a manager who didn't want to use him the way he wanted to be used (Gullit) he fell out with him and undermined him. Everyone talks about what would have happened if he went to United, my guess is we know exactly what would have happened, the same as Van Nistelrooy. He would have scored a load of goals, but United wouldn't have quite been the same team with him, and in the end he would have been moved on and United benefited without him.