If Gandalf was the wisest of the Maiar, why was Saruman known as the wise...
Well, even if one isn't the wisest one can still be wise. Also, wisdom can come in different forms... Saruman's Elvish name was Curunír, "Man of Craft" (according to "The Istari"). The name "Saruman" meant the same thing, in the language of the Northern Men. It may be that the areas of Saruman's wisdom were more easily demonstrated than the areas of Gandalf's wisdom... also, I think Saruman was a lot less reticent in letting other people know how wise he was; Gandalf tended to hide the extent of his own abilities.
... and why was Saruman the leader of the Istari?
Now that's an interesting question... "The Istari" hints that Saruman may have been more powerful than Gandalf; "power" and "wisdom" are not the same, and being the wisest does not necessarily imply being the most powerful, or vice versa. Or, maybe it was just that Saruman was more eager to lead. Unfinished parts of "The Istari" tell how Olórin (Gandalf) didn't really want to go to Middle Earth, and basically had to be ordered into it, while Curumo (Saruman) volunteered to go. Also, it seems Saruman always wanted to be in charge, while Gandalf didn't: Saruman accepted leadership of the White Council after Gandalf refused Galadriel's nomination. Perhaps a desire to lead, to be in charge, was the flaw in Saruman's character that lead to his downfall?