For the original series, it has to be the second or third (I forget which) Wily Stage from Megaman 2, where you have to destroy the orbs with Crash Bomber. One miss and you're pretty much hosed.
For the X series, I'd have to say X6's Nightmare Queen in "X-Treme" mode. Rotates around the room almost faster than you can move, and is invincible except when the core leaves its armored DNA-ish body, forcing you to dodge its attacks while blasting and slashing your way to victory at the beat of the most awesome tune ever recorded (with possible exception to High Max's theme).
Any others?
Mega Man 1's Yellow Demon.
To this very day I have still only beaten the bastard three times.
one of the toughest fights for me was MMBN2's quickman, it took me like, 100 tries.
out of the X series, i'd say X4 sigmas final form with Zero.
(that's a mouthful.)
he was Easy with X but i just can't seem to beat him with zero.
Yellow Demon: Just use the select-pause cheat with Thunder Beam equipped. Easy win. Also great against Copy Megaman.
X4: With Zero, it's all about timing... slash and dodge, repeat. Raijengeki might also be of help.
Zero's fire attack (Ryuenjin, I think) works really well in that battle, at least for me. (I went semi off-topic with a few more details in this topic)
Oh shoot, I forgot about that cheat! That would've been helpful for my bro running through the Collection Mega Man 1.
I recently played MM3 on collection, and a lot of the bosses on there are stinkin' hard without their respective powers to beat. Needle Man and Gemini Man in particular gave me a rough time, though I didn't use E-Tanks on 'em at all. Snake Man was tough even with the Spark Shock, which leads me to wonder whether that's even his "weakness" or not.
Quote:
Oh shoot, I forgot about that cheat! That would've been helpful for my bro running through the Collection Mega Man 1.
No it wouldn't. That was removed.
err, wha? From the Mega Man Aniversary Collection? It's still there... That's how I was able to get past that boss... >_>
Funny, I was pretty certain they'd removed it.
Oh well, carry on then.
Just occured to me, which version do you have? I have the PS2 version, maybe it's not in the GC version... (considering that the GC has no select button)
EDIT: Wait, it's apparently supposed to be in the GC version as well (press L+R instead):
I beat the Yellow Devil fair and square. Easy fight. o.o Copy MegaMan got on my nerves a bit though.
Classic hardest fight is Dr. Wily's machine in MegaMan 2. by the time I got to him I didnt have many weapons left, and I was out of Energy tanks. it took forever. =(
X series would probably be X8 with Lumine
has to be Kaiser sigma for me, if i didnt have the golden armour or the sabre i might never have beaten him
I'll have to go with Wily's saucer from Rockman & Forte.
fregging teleporter...
No, wait, any fight on the Gameboy Megaman games.
The combination of horrible control and tiny screen made any boss fighting impossible.
I finally beat MegaMan 7 just the other day, if you think the teleporter on ANY game is evil, just look at MM7
MM7's probably the easiest of the Megaman games.
...you're kidding. 
I find MM6 to be far easier... X4 as well when not referring to just the classic series.
Yeah, I beat MM6 in two days. I got it for my birthday a few years ago. I sat down and went through all the Robot Masters one day, and went through the fortresses and all that the next.
Toughest boss fights? I dunno, nothing is really jumping to my mind. Though, I remember always getting handed by Byte when I used to rent MMX3 when I was younger.
I can beat MegaMan 1, 3, 4, 5, 6, and 8 in one day if I try hard enough o.o;
MM2 and MM7 have those annoying lazer / fire sections where you die if you go the wrong way or dont move fast enough y'see, makes em way toughter IMO
I recently played through the Megaman Anniversary Collection on PS2 in normal mode. Here are my pics. I wasn't using the rapid fire option either
Megaman 1: Elecman, could never beat him without rolling cutter, Yellow demon used to be a pain in the ass but i got the rythm down, same thing with Iceman, nowadays it's like it's so obvious how did I lose so badly before.
Wiley's final form is a @#%$ without Fire, or Ice Slasher.
Megaman 2: That damn dragon still get's me time to time, with his horns killing me instantly or pushing me off the ledge. That stupid alien at the end, if you don't have bubble lead your screwed.
Megaman 3: The only two who really got on my nerves were the returns of Quickman and Crashman. Quickman just stomped the hell outta me from the start.
Megaman 4: Ringman!!!! I used to think Pharoah was the toughest but Ringman maybe the single toughest robot master of the original series, for me anyway.
Megaman 5: Napalmman was a little tough but got the rythm down on the third try.
Megaman 6: No problems.
Megaman 7: Turboman, Freezeman, and slashman along with Wiley's final form were almost too much to get through.
Megaman 8: Astromans stage and wiley's fortress.
I am looking for new recruits to join the Megaman Anniversary forum over at www.IMDB.com. We have votings for favorite characters, and I am currently running a character battle. But I need more participants to keep it interesting. We had almost 20 posters but all but 5 have not come back in at least 3 months.
Megaman-yellow devil
Megaman 2-any boss
MM3-any boss
MM8-Grenade Man
X5-Shadow devil
X-Storm Eagle
X-Sigma(Final)
X-D-Rex and Rangda Bangda
And one hardests bosses are
X-Bosspider
and
X-Sigma(Final Form)
