I too enjoyed doing Spy missions solo. The rare time I feel like you get to appreciate WF at a moderate pace. But once you know the layouts they're a little less fun to do. Operators also trivialize a good chunk of the obstacles now. Or you play Loki.
But Survival is straight up the most enjoyable mission type to me. Bathing in the endless murder stream is to me the best part of WF outside of the movement. Exterminate, Capture, Excavation, Assassination, Interception, Sabotage...all the other missions are things to be blown through as fast as possible. Mobile Defense is spicey for 3 minutes three times and it's over, and Defense has the long pauses between enemies or when one of them glitches out and you have to hunt them down.
Survival though.....that's where the game is the spiciest. At 45 minutes on almost any map, the game is throwing everything it can at you and the spawns are so thick there are guys to kill every time you turn around. Dodging, rolling, gliding, all while shooting or meleeing your guts out, it's glorious. It's the only time I've ever felt the game even remotely comes close to having a sense of balance. Everything else is either far too easy and not threatening, an annoying boss fight, or level 100+ enemies where everything but the most broken combos fall apart. "At level" survival once you've survived enough waves to get things rolling is where how good your gear is and how well you move come together and you actually get in to a flow state. The only downside is the time you have to invest in the mission to get it to that point....but high enough level gameplay keeps the challenge semi-relevant until you get there.
By comparison I find pretty much every other mission type a necessary waste of my time or wholly focused on just getting to where you're going as quickly and stylishly as possible. No other mission type lets me get into that WF flow state for a satisfying amount of time except Survival or very, very high level endless variants, all of which will usually have a failure point other than being overwhelmed as part of the bargain.