After defeating an end boss, MegaMan is rewarded with an ability (named after the defeated foe) that can be used to exploit another boss' weakness. Thus, the easiest way to beat each of the bosses is to fight them in a particular order. Here's the best order:
For all weapons and items plus four energy tanks, enter the password "A5 B2 B4 C1 C3 C5 D4 D5 E2".
Mega Man 2: Long Jumps
If you pause in the middle of a jump, you'll reset Mega Man's downward velocity. If you press Start rapidly at the height of a jump, you can coast for long distances. This also makes Mega Man "phase out", causing most bullets to fly through you without harm.
Mega Man 2: Password for Dr. Wily's Castle
To skip to the fourth level of Dr. Wily's castle, enter the password "A1 B2 B4 C1 C5 D1 D3 E3 E5".
Mega Man 3: Invincibility
If you do the high jump trick press RIGHT on controller 2 while playing and jump into a pit, you do not die, and can walk around in it. Once in the pit, keep jumping in and out, and eventually your energy will drain but you will still be alive. You are now invincible. Jump out of the pit and your energy will stay down... you do not need to keep holding RIGHT on C2.
Note: If you pick up any energy items or use an Energy Tank, you will no longer be invincible...and you can still die if you fall on spikes. If you have trouble using it, try releasing RIGHT once you are in the pit for a second.
Mega Man 3: Passwords
All weapons, All Items, 9 ETs, no Doc Robots
Blue: A3 B5 D3 F4 Red: A6
All weapons, All Items, 9ETs, Dr. Wily Stage 1
Blue: A1 A3 B2 B5 D3 F4 Red: A6 E1
Spark Man defeated
Red: C5, F4
Snake Man defeated
Red: C5, F6
Needle Man defeated
Red: D3, E6 (Rush Jet Included)
Hard Man defeated
Red: C4, E6
Top Man defeated
Red: A3, C5
Gemini Man defeated
Red: B5, C5
Magnet Man defeated
Red: C5, F5
Shadow Man defeated
Red: C5, D6 (Rush Marine Included)
Mega Man 3: Slow Motion
To slow portions of the game to a crawl, hold Up on Controller Two while playing.
Mega Man 4: All Weapons and Items
For all weapons and items, enter the password "A1 A4 B5 E2 F1 F3".
Mega Man 4: Free Pharaoh Shots
To kill with the Pharaoh Shot without using energy, power up a shot, then hit something while the shot is still building. Press Start then resume play and power up the shot again. Repeat as needed.
Mega Man 4: Secret Weapons
In Dive Man's level, after beating the second whale, there should be a hole nearby. Fall in it and dodge all the spikes to find the Wire Power. This power allows you to hang on the ceiling and hit enemies for fair damage. In Pharaoh Man's level, instead of entering the first hole, use the R.Coil to jump over it. Go past the desert to find the Balloon Power. This allows you to release balloons that you can jump on.
Mega Man 5: All Weapons and Items
Blue: B4 D6 F1 Red: C1 D4 F6
Mega Man 5: Extra Continues
On the first level of DR WILEY's CASTLE you'll find an energy tank. Get it, then kill yourself and continue the game. You can continue this process until you have nine continues.
Mega Man 5: Level Passwords
Charge Man Defeated Red: C1, D4, F6; Blue: B4, D6, F1
Crystal Man Defeated Red: A5, D2, E3; Blue: B1, B3, E5
Dr. Wily's Lair Red: C1, D4, F6; Blue: B4, D6, F1
Gravity Man Defeated Red: A5, B1, F4; Blue: C4, E5, F1
Gyro Man Defeated: Red: A5, B1, E3; Blue: B3, E5, F1
Napalm Man Defeated: Red: A5, C1, E3; Blue: B3, E2, E5
Star Man Defeated: Red: B1, B6, F4; Blue: C4, E6, F1
Stone Man Defeated: Red: C1, E3, F6; Blue: B3, D6, E2
Wave Man Defeated: Red: B1, B6, D3; Blue: B3, E6, F1
Mega Man 5: M Tank 1 Up Trick
During the game if you have an M Tank and have full energy for life and all weapons pause the game and use an M tank. All of the enemies will be turned into 1 ups. note- does not work on bosses!
Mega Man 6: All Weapons and Items
For all weapons and items, enter the password "B6 D4 F2 F4 F6".
Mega Man 6: Boss Sequence
The order you should defeat the bosses in:
Knight Man
Centaur Man
Wind Man
Flame Man
Blizzard Man
Plant Man
Tomahawk Man
Yamato Man
Mega Man 7: All Weapons and Items
To start the game with all weapons and items, enter the password "7 2 5 1 5 8 4 2 2 8 4 7 6 1 3 7".
Mega Man 7: Final Stage
To skip to the final stage of Dr. Wily's castle, enter the password: 1415 5585 7823 6251 and press Start.
Mega Man 8: Animation Test
To access the Animation Test and extra voices in the Voice Test mode, highlight "Bonus Mode" on the start screen, hold down L + R and press Start. The animation test will now be available allowing you to watch all the FMV segments in the game (including the ending). Also, the Voice Test will now have voices for all the Bosses, including Cutman and Woodman.