Custard is probably the most popular foodstuff used to get messy. The various consistencies can be altered to the users liking and sometimes taste. The usual yellow colour makes it stimulating to the eye and fun to watch others wallow in it if you like the colour yellow. Due to its viscosity, it can only be spread so far so a proper full body covering needs a few good litres of the stuff to prove sufficient, again, depending on the user's preference.
Thick custard runs slowly over the person it's poured over--covering them without all running off completely.