I'm a huge Mega Man fan, and I can confirm that 3, 4, and 5 on GB are absolutely fantastic. 2-4 are like mash-ups of the NES game with the same number and the next Nes one. For example, Mega Man 3 GB has 4 bosses from Mega Man 3 and 4 bosses from Mega Man 4. Mega Man 5 GB is a completely new, original experience.
2 is the weakest of the GB games. It can be beaten in about 20 minutes, and the game is so bad that Keji Inafune has actually apologized for even making it. It is only worth it if you are a huge fan, like myself, and want to experience it, knowing it's bad.