That'd be a very rare occasion for me, I pretty much always have a favorite. Khabib vs McGregor would actually be a good example for a fight where I felt like I'd be 100% fine and happy with either result. Khabib vs Tony could be another one of those, hoping it will happen and become a classic.
The most exciting and most popular example would probably be Robbie Lawler vs Rory MacDonald 2 from UFC 189. I kept switching between sides during the action until between rounds 4 and 5 I just said screw it, whoever finishes the other this next round is the deserving victor and I'm fine with it.
Then there are fights where I would've been fine either way but didn't like, best example probably being Anderson Silva vs Demian Maia.