I wouldn't recommend 2. The game is the most punishing in the series, as if you die you lose all of your E-Tanks. The Wily stages in particular are among the worst, if not the worst in the series as well.
Aside from the Yellow Devil, 1 isn't bad if you know what you're doing (it's probably the easiest game to speed run). It's not a bad place to start, but do be prepared to die a lot if it's your first time playing.
3 or 5 I'd say are the best place to start for the original 6 games. 3 is fairly well rounded in terms of difficulty, while 5 is pretty easy but still entertaining due to it's level design (such as Gravity Man's stage).
9 is the best game in the series IMO, and unarguably has the best weapon selection in the Classic series. But it is one of the harder games as well.
10 is one of the meh titles overall. There is an easy mode, but it does spoil players. There is also a Challenge mode that teaches game mechanics as well.
7/8 are both good games (they're amazingly underrated). But they do differ from the rest of the series due to larger sprites, levels, and hidden secrets, so they're not that great for starting out with. Avoid Bass unless you're a Pro Mega Man player.
IMO: 3/5 > 1/6/9/10 > 4/7 > 2/8 > Bass