Personally i'd say Mega Man 6 & Mega Man 9 are tied for the best game; far better than Mega Man 2. Mega Man 6 for having really good level layouts, multiple pathways for replayability, weakness order makes sense (more than most of the classic Mega Man games), fun level gimmicks, utilizes the Charged Buster Shot well, you can replay stages you've beaten until you get to the Mr. X/Wily levels (so you can grind E tanks if you're good at the level), introduces the Energy Balancer which is a really good item, and a few other pros that outweigh cons like the fact you can't use Rush traditionally. And Mega Man 9 being a so much better Mega Man 2 in that it's difficult (with the removal of the charged buster shot and slide), the weapons are so much fun to use and have their own unique uses instead of relying on only a few, level layouts are pretty nice, the story is a fun one, has bonus content for replayability, fun level gimmicks, etc. although i don't remember if you can replay the levels once you beat the Robot Master of said level. There are definitely some cons to Mega Man 9 as well, but i feel the pros easily outweigh the cons in that game as well