Depends on the mode, really.
In Turf War, Urchin Underpass is my favorite; it's a fun tug-of-war generally, but I like it most because it's easy for me to reach the center, then take the corridor into their territory, splatting people if I can, sometimes just going back to the center through their opening and covering that (with varying success). Saltspray is fun too because there's so much ground to cover and I can't get spawncamped, but not being able to ink everywhere will always annoy me, and while I like Port Mackerel for its corridors and elevated areas, I just can't keep track of everything like I'd like to (I have the same problem on Saltspray). Conversely, Arowana Mall is too easy to sneak into the enemy base on and the center gets really congested generally, Walleye Warehouse is too easy to get overrun on, and Blackbelly Skatepark is just too open for me (so is Walleye, for that matter).
In Splat Zones, Underpass is still a fun struggle, and Arowana and Walleye are still not that great to me, but the rest change for me. Now that there's a definite point to get to (instead of just "cover EVERYTHING"), Saltspray is somehow more hectic, which makes me like it more (though I swear it feels like that lower area should've had a Zone too). Blackbelly also has a more definite place to fight over, so the openness isn't a problem anymore, but now I'm a bit lost, since getting the central platform still doesn't feel that important unless the other team has it. Port Mackerel became a fast favorite in Splat Zones, since I can get around quickly (Seekers, throw one and follow the trail to replenish my ink and it'll splat anyone it hits, which is nice too) and harass enemies at their zone easily while still being able to defend either zone if needed (thank you, .52 Gal Deco).
So overall, I think my favorite for now is Port Mackerel, mostly for the varied ways that I feel like it can go (the corridors are worth a surprising amount, especially the ones to the left and right of each spawn point). I feel like it's always up in the air when I'm on this level, unlike a few others (Underpass comes to mind at times, so does Walleye in general) where I keep getting barely near the center before getting splatted.