Lauzon/Miller. 
Gus v rumble
Diaz v silva
Lawler v ellenberger
I'm sure theirs plenty more
But these are of the top of my head 
Franklin Liddell.
I know it bothered Rich that he was Chucks last fight and essentially retired him.
hated seeing both go in there to fight.
Penn vs. Pulver
Cerrone vs. Diaz
Condit vs. Diaz
Mir vs. Cro Cop
Diego vs Gil
kennyfrommd - Frankie/Maynard II was what sold me as a Frankie fan. Before that, I thought he was just another solid contender, but watching him stumble around the ring and struggle back to his feet time and time again made him look like Rocky. He forever earned my respect with his gameness in both of those fights.Frankie fan for life! Unreal heart!
Bigfoot v Hunt
Shogun v Hendo
WEC 43: Cerrone vs. Henderson
Benson vs Cerrone was probably the most recent one where I didn't know who I was rooting for. Was probably leaning Cerrone but was happy with either outcome.
Condit Vs. Diaz
Cro Cop Vs. Fedor
The Rock Vs. Austin
Siaz vs Condit but that changed after the 3rd round
T1LT - Siaz vs Condit but that changed after the 3rd roundDiaz*
Great thread and so many memorable battles have already been mentioned.
The first fight that came to my mind was Rampage vs Lyoto. Lyoto had lost his belt against a fired up Shogun who managed to do what many believed to be, nearly impossible. And now Lyoto was more motivated then ever to reclaim his throne. Rampage had looked unfocused and in limbo between fighting/movies/pro wrestling, and also, coming of a loss by Rashad Evans.
I love what Rampage represented with his old Bushido spirit, and I could not help but to feel that he was being feed for the impressive force of Machida, (who I become a great fan of as well) But then as Rampage made his entrance with the Pride song, holy shit!
I loved the outcome as well, Machida impressed, even in defeat :)
https://m.youtube.com/watch?v=Zbo3LizegQM
Gracias Blue!
Every Chuck v Randy fight
Quarry v Franklin
Chuck v Franklin
Pulver v Lauzon
Shogun v Forrest
Lauzon v Varner
Fedor v Nog
Shogun v Arona
Wand v Hendo
Wand v Rampage
Frankie v Cub
faber v Aldo
Handy vs Nog
Some future ones too
Silva Diaz
Bj Diaz
Aldo mcgregor
Hendo moose
Hendo shogun
Machida rockhold (slightly towards machida)
Werdum vs hunt
Diaz vs Penn
All fairly recent...
Cain vs JDS
Rumble vs Gus
Machida vs Munoz
UK Rampage - Handy vs Nog
For sure this one.
This was the first fight that comes to mind. I was so worried before that fight, I knew someone had to lose, but during that fight I was SO proud of both fighters at different points in that fight. Randy was in some bad positions but showed heart the entire time, as well as great sub defense!
The 2nd one for me was already mentioned, Gilbert Melendez vs Diego Sanchez.
I thought and wanted Gil to win, but my god the heart Diego showed, some point after the 2nd round I was rooting for Diego, "COME ON DIEGO, COME ON, YES YES YES"