It varies wildly for me- I'm transfemme so I naturally lean more toward a feminine outfit. Not got much deep mud immediately near me but have had some chance to play before.
Working from the feet up, I'll often wear something with a heel because of how much more awkward they are, how they unsettle my balance and force me to struggle more. Boots are preferred as they're less likely to slip off my feet, though something with a strap like Mary Janes can be pretty nice too.
Legs depend on the outfit; I've tried jeans/denim, they are fun and help further restrict you as they get soaked and covered in muck, leggings or tights with a skirt can be good fun. Getting deep enough to have the skirt end up floating on the surface somewhat could be very fun.
For the top, it also varies.
I've considered doing an equestrian outfit if I can find the parts cheap enough; jods, blazer, blouse and riding boots. Much fun :3
I do also have plans to wear a maid outfit and petticoat to play around in an estuary sometime later this year. I bet that due to the awkwardness of the outfit that once I get much past the knees I'll start to get stuck, and be going nowhere but down by the time I'm thigh deep