We do this every so often, it's time to do it again.
Pick ONE. Write it here.
"DO IT NOW" * Pride Voice *
The first one that comes to mind is
Diego vs Clay
But if I sit around and think about it.. There are many
Hendo/Shogun 
Saku vs Newton
Told ya lol
RickStorm -Saku vs Newton
Told ya lol
Sanchez/Guida was the first fight that made me think "holy shit I love this sport" (I'm fairly new to MMA). Hendo/Shogun is definitely my favorite fight.
Not a great representation of skill but god damn the punishment those two dudes put on each other. Just standing there pummeling each other.
Wanderlei vs. Rampage (Pride)
The most excited I ever felt watching a single fight was Nick Diaz vs Gomi.
For the year I have to go with TKZ vs Porier.
But there have been so many great fights that I'm sure ill change my mind in 5 min.
Nog-Sapp. My heart almost exploded out of my chest. That was frightening, and then Nog did Nog
Tyson Griffin vs Clay Guida
Nick Diaz vs Takanori Gomi
Silva vs Sonnen 1.
Fans still don't understand it's significance or specialness at all. 
Gomi-Diaz.(Nick)
Diaz/Lawler
Nick Diaz vs Paul Daley
Aldo vs Hominick
Would have to think about my favorite fight.
The moment that comes to mind is when GSP beat Hughes.
Shogun vs lil nog Wandy vs Cung le
Fedor vs. Cro Cop
Honourable Mentions:
GSP vs. Hughes 2
Royce vs,Sakuraba
Cro Cop vs. Wanderlei
Wanderlei vs. Hendo
Hughes-Trigg 2 was one of the single greatest moments in the history of sports.