gm_goldencity and gm_daylight(also download navmesh cuz it does not contain it) is fun experience for me

If you're willing to do some setup, Everpine Mall almost feels like it was made for this.

I find that maps designed with npc battles in mind tend to work right out of the box.

Unironically, rp_animeplanet. Lots of areas to take cover, big enough map with some interesting layouts, it's actually one of the best that I've tried.
That, and gm_shambles.

rp_nycity was always really fun for me. The map is big enough to where tanks and dropships are viable, while also allowing alternative methods of transportation to get across the two islands. There's the subways which might end up in a hold-out type scenario where you have to hold your position while waiting for the train to stop by. You could also take the bridge if you're willing to risk the damage from the gunships or choppers.