A jockstrap, speedo, or shorts!!

Levi's skinny jeans, white or denim shirt and denim jacket

Lycra boxer, one piece swimsuit

Probably my fave ever gunging took place in primary coloured tee & nylon shorts as it kinda made me feel like an overgrown gameshow contestant.

Preferably formal wear, specifically suits, tuxedos and uniforms.

Lycra, or waterproofs and wellies

I like to look very smartly dressed and I even spend time fixing my hair.

I second this - I prefer just a t-shirt and shorts as it's nice and casual and adds to the gameshow feel.

A proper tanking in a full German lederhosen/tracht outfit would be fun though

you could always make up an outfit and use a suggestion from what everybody wrote or just wear everything everyone wrote on top of everything than slowly remove all of that off camera while getting messy and reverse what you show by slowing things down while getting messy

Speedos or my rugby kit

Love to look my best. Smart clothing. Dress shirts and dress pants that sorta thing. Feels more naughty getting messy that way.

Jockstraps and combat boots

My birthday suit!

Naked and Birthday Suit only. I find clothes tiresome.

Jeans and over knee or thigh boots or waders. They just hold more slop.

I like to start out fully clothed, and slowly strip down to nothing. I prefer not to get my clothes too muddy so I can reuse them, but starting out naked is kinda boring - there's no sense of anticipation. I prefer to watch people mudding naked, or clothed at first but then naked.

Depends on what I'm getting messy with or what environment I'm in.
What would you be getting messy with in general?

When I'm getting muddy, it's casual gear all the way -- jeans or joggers, rugby shirt, hoodie, maybe a puffa jacket if it's cold. And trainers or wellies.