So after reading so many people complain about inability to have good shootouts, I took it upon myself to find some. I will post them here as I find them, and others are welcome to post their favorite shootout locations.
1. Parking garage at Strawberry
Parking Garages are great locations to host a shootout because you're covered from helicopters. This one is my favorite for several reasons.
For one, the walls are nice and high to provide great cover and there's lots of space to move around in, but that also means cops have lots of space if you allow them to become mobile.
There is only one entrance into this parking garage, allowing you to hold your position very well. Behind you is a covered wall, so you do not have to worry about taking too much fire from behind.
On the roof of the garage there is a ramp you can take if things get far too heavy. You could also jump to the roof below, then down again to get into a nice tight alley. (You may take fall damage)
My favorite location to hold my ground is on the Third level at the end of the ramp behind the pillar, the police tend to all stick on one side so it's pretty easy to pick them off as they come around the corner, and since you're in the air, you don't need to worry about being shot from behind.
I will be adding future posts to this thread with even more locations as I find them, I only just began my search and so far It's paid off well. Also please include your favorite shootout locations!