[MXZA]

X: Boomer Kuwanger - cool name and weapon
X2: Crystal Snail - stage music and level design
X3: Toxic Seahorse - stage music, level design and mini boss
X4: Web Spider - level design, boss fight and weapon (cause it's always funny to see S. Mushroom scream like a girl)
X5: Duff McWhalen or Tidal Makkoeen - stage music, weapon and boss fight
X6: Rainy Turtloid - level design and weapon
X7: Wind Crowrang - level design, weapon and stage music
X8: Dark Mantis - weapon, stage music and level design

[Princess Luna]

I believe their is already a section for this, though I may be wrong.

[Sigma Zero X]

Favorite bosses:

All Mega Man X games: Sigma

Reasons: Fierce fighting moves, charismatic, BA, and cool all around in my book. Just can't really have a Mega Man X game without him, maybe except Command Mission

As for the other bosses besides Sigma -

X1:

- Chill Penguin (fun boss fight)
- Storm Eagle (fun boss fight)
- Boomerang Kuwanger (cool stage music, great personality)

X4:

- Frost Walrus (fun boss fight)
- Split Mushroom (fun boss fight)

X7:

- Red (really fun boss fight on Hard that made me love X7 a lot more)
- Soldier Stonekong (a simple boss fight IMO, but enjoyable nonetheless)
- Tornado Tonion (boss fight that helped me learn about X7's mechanics)

X8:

- Gravity Antonion (fun stage and reasonable first boss on Hard mode)
- Earthrock Trilobyte (cool stage to gain some metals)
- Lumine (a very fun boss fight on both forms)

Command Mission:

- Duckbill Mole (cool fight, and worth gaining Absolute Zero, although I still like the Power Charge earned from him more)
- Rafflesian (another cool fight, and while the Ultimate Armor is a great reward, the Sub Tank parts are far more important for me)
- Epsilon (a very intense boss fight with Metacrush, but earning Sub Tank parts from him makes the fight more fun and worth it)
- Redips (another very intense boss fight on both forms)
- All tails clan members except Fivetails (Onetail - Ninetails are interesting fights, but the Self-Destruct move from Fivetails can DIAF)

[Ultimate X]

Megaman X: None

Megaman X2: X-hunter stage 1 & 2 (music is epic)

Megaman X3: same as MXZA

Megaman X4: Jet Stingray ( I love how do they transport ^.^ )

Megaman X5: X vs Zero ( Epic music, Epic boss fight )

Megaman X6: Infinity Mijinion ( F*****G epic music)

Megaman x7: Never played it xD

Megaman X8: never played it xD

[ProtoBlues98]

X1 - Chill Penguin because his music is fantastic you get the Dash Boots, and he has a fun fight! (Unlike Sparky there)

X2 - Does Agile count? I would say so. I loved his unique personality and his amazing fight concept. Even with his weakness he is still a challenge.

X3 - meh, I wasn't crazy about X3, but I guess Toxic Seahorse. Not really any reason in particular, except he melts and reforms.

X4 - Cyber Peacock. I LOVE places that present you with challenges and then judge you on it in any game! (I wonder what my boss wil be for X8?)

X5 - Mattrex, just because he is a FLAMING DINOSAUR. Sure his stage was annoying and getting that capsule was tricky, but still. FLAMING. DINOSAUR.

X6 - High Max. FFFUUUU-just kidding. Beating him before the end of the game in a "Phantom Realm" with Zero is harder than Mr. Clean's sex life (makes no sense I know) but I love a good challenge...especially one that has me stuck on it for hours at a time. The only thing that makes it worthwhile is the music. (hard tie with Dynamo)

X7 - Flame HyanBURNTOTHEGROUND!BURNTOTHEBURNTOTHEAHAHAHAHAHBURN!...Yeah, THAT''S why.

X8 - Optic Sunflower look at X6 dummy.

What do you guys think? Also if Agile doesn't count I'll say Wheel Gator...for the music and he is a cool boss. And if High Max doesn't count I'll say Rainy Turtleoid. A huge boss with a stage with TONS of hidden item that I got by utilizing invincibility frames (even the armor.)