Well, I consider myself basically a master of the mono-type (I've created a mono type team for every single type). I think it's no surprise to know that the most powerful types for the mono-tier are Water, Steel, and Dragon (in that order, probably). The weakest is definitely fire (it stands absolutely no chance against water, dragon, ground, or rock. There's just not enough variety among fire types).
The most important thing to keep in mind in the mono tier is to find things that cover your weaknesses. For instance, poison's main banes are psychic and ground, so it's important to utilize Drapion and Crobat/Gengar/Weezing (I usually use two of the three. Generally Crobat and Gengar).
Water is the best simply because there's so much variety. With Drizzle it's just not fair, and even without drizzle your opponent won't stand much of a chance. Even mono electric teams will have a lot of trouble getting past your Swampert/Gastrodon/Seismitoad/Quagsire (every mono water team uses one of those). The only mono type team that will usually win all the time is grass, but even then, a well played Cloyster will only lose to Mach Punch Breloom.
I could speculate for hours on how fun the mono tier is. Each type is fun to play in its own right (except fire. It's so frustrating. I can hardly ever win.)