In my opinion, it has to be Dragon type. In several Monotype Tournaments I have been in before whenever I pick Dragon, 9 times out of 10, I win. In the most recent Mono-Type Tournament hosted by [supporter]ForeverDash[/supporter], Miss Doronjo destroyed all of the opposition with her Dragon Team.
Personally I don't think there is a best type, however there are types that I find more useful than others. For example, I like to use Steel types quite a bit due to their useful resistances, great defenses, and wide variety of support moves. I also like to use Dragon types for power as they are the most powerful in my opinion. Lastly I feel Water types are one of the most useful due to their versatile movepool with most learning Ice Beam for coverage and their access to very powerful STAB.