maxIndex.swift

let array = [2, 3, 5, 1, 6, 9, 8]
extension Array where Element:Comparable {
    var maxIndex : Int? {
    // Thanks Mike Ash, Jacob Bandes-Storch, Bas Broek
    return self.enumerate().maxElement({$1.element > $0.element})?.index
    }
}
array.maxIndex

// Update: Thanks, Jordan Rose
// http://twitter.com/UINT_MIN/status/624274556098842624
extension CollectionType where Generator.Element: Comparable {
    var maxIndex : Index? {return self.indices.maxElement({self[$1] > self[$0]})}
}

👍 very cool!