Here's what Emily Post says:
{Rehearsal Dinner}
The first toast is made by the dinner's host - usually the groom's father - during the main course. The toast is often met by a "return toast" from the bride's father. Then it's open season - the mother's of the bride and groom, the best man and maid/matron of honor, other wedding attendants, family, and close friends.
{Wedding Reception}
.Traditionally, the first toast at the wedding reception is made by the best man. This is the most formal of all wedding-related toasts, and it occurs after all the guests have been provided with champagne or a nonalcoholic alternative. At a sit-down dinner, the toast is made as soon as everyone is seated; at a cocktail reception, it's made after the couple arrives. It's fine if the best man's toast is the only one made. Often, the fathers of the couple will toast each other's families and the marriage of their children. The maid/matron of honor may offer a toast, and the bride and the groom may toast their families and each other.
It's hard to wing a good toast; it's better if you prepare your remarks ahead of time. You'll never go wrong if you keep it short and sweet. The best toasts last a minute or two - three at most - and can even be just a few lines, such as:
I'd like to take a moment to congratulate Mike and Kathryn on their engagement. Let's raise our glasses to a long life filled with happiness and love for both of them. To Kathryn and Mike!