Hmm. I will answer first as a DM, then as a player:
From behind the screen:
My personal favorite by far has been a night fight on a bridge - the players were holding one end of the bridge to keep an undead army from sacking Fallcrest. I think they were 5th level at the time, though they might have been 6th. The bridge was rigged to collapse, and their job was to A) kill as many undead as possible then B) pull back, luring a good chunk of undead forward so that the villagers could drop the bridge when it was full of undead and isolate a chunk of the undead army on the Fallcrest side of the river where it could be destroyed without reinforcements.
They're crouched down at their of the bridge, a bonfire crackling behind them, listening to the sound of skirmishers dying in the darkness somewhere on the far side of the river - after a while, wounded skirmishers start slipping back across the bridge - the last one stops to light the bonfire on the far side of the bridge to increase visibility for the party, but it costs him his life - gravehound zombies pull him down halfway across the bridge and tear him to shreds. When they finish, the 7 gravehounds turn toward the party and lope towards them, at which point the pyromancer wizard lets off his fireball, hitting 6 of the 7 and doing half damage to the 7th and rolling close to max damage (I think it ended being around 130 points of damage total between all 7 of them). With that as the opening salvo, they quickly destroy the gravehounds. That nets them a short rest while the main undead army marches up. After assessing the situation, the undead general orders forward a 50 skeletons - 2 boneshards, 2 flame-throwing ones, and 46 minions. In front of the skeletons shambles a zombie bear with one paw hacked off and replaced with a magical axe (zombie hulk with some customization by me). The defenders lock down the bear, the wizard's flaming sphere and the cleric's consecrated ground move out onto the bridge creating an endless death zone for minions, and a fierce melee engages (complete with surprise flying zombies landing behind them). Finally, after expending most of their resources, the party stands triumphant - bruised, battered, but alive. We break for the week as the undead general rides out to parley with them.
Now, the first part of the fight had just been on my battlemat. When the players showed up the next week, they found that I'd built the entire bridge and both banks out of dungeon tiles, their minis were grouped defensively at one end of the bridge, and I'd set up every humanoid mini I owned on the other side of the bridge to give them a sense of scale on how many undead they still had left to fight. There was noticeably less pre-game banter as they surveyed the situation and exchanged "oh crap, he's going to TPK us again" looks.
Anyway, parley with the undead general is unsuccessful ("Would you like to join my side?" "Do we have to die to do it?" "Yeah, kind of" "Sorry, thanks but no") and they retreat back to their side to face a rush of zombies, this time with the general staying in the middle of the bridge and buffing his troops from the rear. Just as combat joins, they hear screams from beneath the bridge as ghouls (who took advantage of the parley to crawl under the bridge) kill the villagers who were supposed to knock the bridge down. The entire near side of the bridge is difficult terrain now due to the pools of blood and piles of shattered, burned skeletons. The fighter scrambles under the bridge (and is horrified to find himself facing 3 ghouls with no back-up), the paladin wades to the front line, and the cleric, wizard and warlock unleash hell. Fight lasted many, many rounds and went back and forth several times - finally the paladin managed to fight his way to the undead general and bull-rush him off the bridge, at the same time as the cleric rolled a crit on his athletics check to swing under the bridge and save the fighter from certain death (the fighter had just been coup-de-graced twice and was 2 hp away from negative bloodied).
After that the NPC wizard sets the bridge on fire, they collapse the bridge sending another 100 or so undead into the river, big heroes etc. All told I think they personally knocked off 80 + undead in those 3 fights, along with another 100-150 taken out by the bridge. Of course, that just meant Fallcrest's militia was outnumbered 3 to 1 instead of 4 to 1, but that's another story . . .
Worst fight as a DM so far - I've had a handful that ended up not being interesting for various reasons. I'd have to say the worst one was the time I TPKed them (final fight in KotS). They were missing a couple players for that session, I didn't scale the combat down correctly, and then they had 2 turns in a row of horrible tactics and horrible luck, and suddenly the fighter has been dragged into the portal, the warlord is unconscious, and the rogue is standing there bloodied and out-numbered 3 to 1 . . . not quite how I planned it, but sometimes TPK happens.

Oh well, it ended up being a great excuse to run my zombie horror game ("some incompetent group of heroes didn't stop Kalarel! There's zombies everywhere! Run!")
As a player:
My personal favorite fight was the one where my warlord was on the wrong side of a nasty set of spiderwebs, in negative hp, had two failed death saves, and the rest of the party had no way to get to me before the spider did. I had pretty much resigned myself to my death and was working on new character concepts while everyone else took their turns, when it came around to me and I rolled a nat 20 on my death save with my lucky red d20. It's not all that, but it felt great to pop back onto my feet, and I've been very attached to that character ever since.
Worst fight would probably have to be in the game where I have the wizard - we'd just hit paragon, and were wandering down a tunnel some where when we got randomnly ambushed by a motley collection of slaad and destrachan. It was a crappy fight on every level - bad encounter design, horribly boring terrain, poor party cooperation, fight went on an hour and a half longer than it should have . . . gah that was an ugly night.