The 'Mark as Best Answer' feature has yet to be made to work by AB's techies. (I say 'techies' but I'm fairly confident that AB's system is maintained by a twelve-year-old kid who accesses the site's server from his smartphone while riding his skateboard!). As has been pointed out, it fails to deal with the situation where, after clicking on the button, someone else posts a better answer!
We used to have a 'star rating' system for questioners to respond to answers. If I recall correctly, three stars meant 'thanks very much; you've solved my problem', two stars meant 'sorry, that didn't help, but thanks for trying' and one star meant 'why did you bother to answer?'.
Simply saying 'Thanks' (as nice people like Lankeela do anyway!) would seem to be the easiest way to do things ;-)