For me, it has to be Frank Mir vs Noguiera 2. Frank went from being nearly KO'ed and choked out, to submitting and breaking the arm of the best HW BJJ player in MMA. That was crazy.
What you got? 
Royce catching Ken with the choke, and Ken smacking the mat over and over has lasted the test of memory through time, for me.
Nog submitting Sylvia
Penn Hughes 1
Silva Franklin 1 with Hughes and Rich's wife watching.
Fedor Crocop at the time.
Serra GSP
Bisping Rockhold
Dave Kaplan on TUF getting KOed after claiming to be unKOable 
Robbie v Rory stare down
Vtrt armbar on Jones
Diaz v Conor
Conor v Aldo
Loads
Arona laughing in Wands face then getting destroyed by Shogun. Fedor VS CroCop on the same card.
Tough question..
Watching UFC 1 as a kid absolutely changed my life, and Royce was a hero of mine for a while.
The Superfight between Shamrock and Gracie was a big deal. People say it was boring, but I was on the edge of my seat. My 2 original favorite fighters pretty much stalemating each other.. And I had an even higher opinion of Shamrock after that fight.
The Superfight between Shamrock and Severn. Between how Ken was beating up on Royce in the "overtime" period of there fight, and the way he out muscled and subbed Severn -- Ken Shamrock became IMO the best fighter of those early UFCs. I had expected that to be a competitive fight, and Ken dominated.
Pretty much all of the Pride GPs were on another level for me as far as sporting events. I would compare my excitement to my favorite Superbowls. Pride was the peak of MMA IMHO, and nothing has come close since.
One thing that stands out most to me is the loss of Evan Tanner.
His MySpace was great and through all of his problems he did so much for so many.
He was a good fighter and seemed a better man outside of the ring.
The rise and fall of Mark Kerr is also memorable.
Ken/Tito 1 was memorable as far as disappointment. My favorite fighter vs a guy I thought he should wipe the floor with. Ken looked like shit in that fight, yet managed to take the beating with dignity. He got my hopes up when he buckled Tito'd knees with a punch. But other than managing to escape to his feet a couple times, that was his only moment in the fight. Its important to remember that up until that fight, Ken had never "really" had his ass kicked. He was kicking Fujita's ass before he lost, and the Frye fight was an epic battle that was rightfully a draw IMO. But Ken showed his age against Tito, and the bad guy won. It was the beginning of arguably the worst fall from grace in MMA history. -- To this day, I maintain that a 90s version of Ken Shamrock destroys Tito.
So that was memorable in a negative way.
Nog armbaring Crocop after getting beat up and shut down the whole fight.
stringtheory -
Good post.
I have to agree about Penn\Hughes and Nog\Schaub.
The Hughes fight made me into a BJ Penn fan forever. Prior to that, I was not a big fan. I thought he was cocky (he was), and wins over Uno were not exactly the stuff legends are made of IMO. I was picking Hughes, and was fairly confident he would smash Penn (if a was a gambling man, I would have bet on Hughes). When Penn came out and dropped Hughes, I was on my feet. Penn had challenged the dominant champion of a higher weight class, and he got it done. He every bit backed up his cockiness, and made an instant fan of me.
Big Nog/Schaub was one of my happiest moments as well. Not is one of my all time favorites, and still the second greatest HW ever IMO. But he was obviously declining at this point. When he KO'd Schaub, I started rapid fire 1-2 punching the back of the chair I was sitting backwards on. This abrupt outburst prompted some minor scolding from my wife LOL. But yeah, that was a great moment. A privilege to see one of the all time greats het another win.
I'll also add Hendo/Bisping to the list of happy moments. My brother and law and I were heavily rooting for Hendo (who had been among my favorites for years). Now, I'm not the type of guy to wear MMA related clothes very often. But my wife had bought HENDO t-shirts for us that she ran across the day of the fight. So of course we wore them, and the celebration was very loud to say the least. Bisping had been talking shit.. But Hendo shut him up with one of the greatest KOs of all time. EPIC!!
As a fellow Englishman has to be Bisping winning the MW title, after losing to Rockhold in 14 I thought he'd never get a chance to fight for the title, but stepping in at short notice and avenging the loss with a first round KO? Nothing sweeter
Another one...
Couture/Sylvia.
Couture was already one of the greats, and a 2 division champ. But after Chuck proved he had his number, twice; a return to HW against a fighter like Sylvia was (literally) a tall order.
Could Randy surprise us again?? Umm,yeah!
I was on my feet as soon as he came out and dropped Sylvia -- and I was laughing like a kid on a roller coaster for the rest of the fight. I think that's my all time favorite Randy Couture fight, just because of how and when it went down.
Hughes/Penn 1 and 2 were epic. BJ's walkout for the second fight was awesome.
GSP vs. Hughes UFC 65.
GSP had to fight some tough guys to get back to Hughes for a title fight.
My neighbour called the cops on us that night. We were cheering so loud.
Hendo Koing Biping
Also.
Annnnnnnnnnnnnd NEW!!! Ufc welterweight Champion of the worrrrrrld... "RUTHLESS" ROBBIE LAWLEEEEERRRRR
Rhodes - Ettish definitely the most memorable for me, except that I actually don't remember a large part of it. They say head trauma will do that.
Nagashima v Aoki, remember staying up with my buddy Troy watching that live @ like 5am.
Sylvia v Couture, went totally ape shit when Randy dropped him in the 1st
Crocop v gonzaga, :( :( :( :(
Noons v Diaz 1, didn't give kj a chance, he was sharp that night (so to speak). I've been a fan ever since.
Scott Smith v Cung Lee 1, was a huge fan of Smith after his crazy fight with Sell, really wanted hands of steel to take Chung's 0 (love Cung Lee too, dude looks great in the right lighting). His fights with Martin, Radach, and Noke were great too. P.S. Still pissed about the Terrell fight, that was bullshit.
Holm v Rousey, for a more recent fight, I pretty much started doing involuntary backflips when that happened.
Diaz Gogoing Gomi.
Frederick Ettish - Rhodes - Ettish definitely the most memorable for me, except that I actually don't remember a large part of it. They say head trauma will do that.
LOL
Voted up!