I know this is going to be a long response, but here's what I have to say about this matter:
Do not overly rely on Metal Blade. It is a great disruption move, but be warned that it can be caught by other players, and you can't use another Metal Blade until that Metal Blade is thrown. It helps to vary up the usage of it as well. Grabbing the Metal Blade helps, as well as Z-Air, which is dropping the Metal Blade, which can gimp recoveries and help combos. It can aimed in multiple directions as well, so use that to catch your opponents off guard.
The Crash Bomber is a good shield breaker, and use that in combination of lemons or the Side Smash (Mega Buster) to break the shield.
Leaf Shield may look useless, but it has a lot of untapped potential. Since Mega Man's pummel (grab attack) is slow and can be easily broken out of, the Leaf Shield's extra damage can help support the grab and throw by adding a little more damage. It can also produce a mini spike that can gimp recovery, but it's risky and you need to know the timing and duration of the move to get it to work.
Mega Man's Smash attacks are really good, but have a lot of ending lag, which can cause some serious problems. His Down-Smash, Flame Blast, is a good punish for roll abusers and a good punish after a shield break. Mega Buster is good for edge guarding and damaging shields, and the move has a lot of knockback, so it can kill pretty early when near and edge fully charged. I don't recommend fully charging it unless you're sure you can get it in, so it helps to use weaker versions, which also have good knockback. The Spark Shock will be your goto Smash attack, as it has the least amount of ending lag and it can help lead into combos. At early percentages, you can lead into two Spark Shocks.
As for his throws, Mega Man's Down and Back Throws will be your goto throws. His Down throw can lead into combos, like Flame Sword or Neutral Air, and his Back Throw is his kill throw. His Up throw is okay, as it can be followed up by an Air Shooter. His Forward throw is his least useful throw, but is does have decent knockback can could be potentially followed up by a Metal Blade.
Mega Man's tilts are pretty solid. Down Tilt (slide) can be used to catch opponents off guard, and his Up Tilt, the Mega Uppercut, is one of his best kill moves. Just keep in mind that the uppercut can whiff and if you miss, it does have a good amount of ending lag, so you're gonna be punished if it misses. His Forward Tilt isn't another thing to write home about, as it's his jab but moving, and those moves can allow opponents to keep away from you, and if you hit up close, it can add further knockback and damage. His Dash Attack, Top Spin, is a solid dash attack that can do a good bit of damage, but can be blocked so keep that in mind.
As for recovery, Mega Man has a wall jump, which can help recovering, and Rush Coil is a great recovery, since it has a lot of vertical height, and decent horizontal distance if you keep moving. Just keep in mind that it is predictable, so don't be surprised if you get spiked or knocked off while recovering.
Mega Man's air attacks are actually pretty solid and useful. His Forward Air, Flame Sword, covers a good amount of space and does pretty good damage to boot, though it can do less damage if not fully connected. His Down Air, the Hard Knuckle, is a safe, but very good spike move. You won't be in too much danger if you miss, and if you hit with the spike, they'll go flying down... unless the character has a good recovery or is at a low enough percent, they can recover so keep that in mind. His Back Air, Slash Claw, is another good kill option. It hits three times, with the first hit having trip properties, and the third hit having a good deal of knock back. His Up Air, the Air Shooter, is by far his best air attack. Not only does it do a good amount of damage, it can kill surprisingly early and can help air combos. It's the closest attack that Mega Man can reliably spam, but don't do that unless you want to troll your opponent.
As far as weaknesses go, Mega Man's close up game is pretty bad, so if you're fighting against characters with aggressive close up games, like Fox, if he gets in Mega Man's space, Mega's gonna have a hard time. Also, characters with reflectors like Fox can counter a lot of Mega Man's attacks, and cuts down a few kill moves, mainly the Mega Buster Side Smash. A lot of his moves, like his Smash attacks, have a lot of ending lag, so if they miss or don't connect, you're going to be punished horribly for it.
Mega Man, however, excels in edge guarding and zoning. He can prolong the game by keeping his distance. It also helps to be a patient player, as the key to winning with Mega Man is patience, waiting for your opponent to make a mistake, and going in for the kill. Here's some combos:
Grab, Down Throw, Forward Air X2
Grab, Down Throw, Forward Air, Grab, Down Throw, Forward Air
Grab, Down Throw, Forward Air, Neutral Special, Dash Attack
Down Special, Grab, Back Throw (or any throw)
Up Smash, Up Smash
Side Special (when opponent is blocking), Side Smash (when shield is broken), Side Smash or Down Smash
Grab, Up Throw, Up Air (use this as many times as your heart's consent)
Grab, Forward Throw, Down Air
Grab, Back Throw, Back Air
Grab, Down Throw, Neutral Air
Grab, Down Throw, Forward Air, Up Tilt
