What specific strategies have the highest win:loss ratio in Uno?
I play the largest number I can and save my draws, skips, and reverses for when they can be used against a winning opponent. I really don't worry about number variety. However, I try to keep a low number of high value cards in my hand, namely reverses, wilds, and skips, in that order, so if someone manages to win I don't give them too many points.

call color: No Strategy (color i have?)

play: 
if hand has X or more Special Cards (SC), 
    play SC(if possible): priority Reverse, Wild, Skip, (Wild Draw-4, Draw-2), 9-0
else if opponent has Y or fewer cards
    if opponent next and hand has Draw-2 or Skip (or Wild Draw-4)
        play Draw-2, Skip, (or Wild Draw-4)
    else if opponent last and hand has Reverse and (Draw-2 or Wild Draw-4)
        play Reverse
    else
        play 9-0, Draw-2, Skip, Reverse, (Wild, Wild Draw-4)
else
    play 9-0, Draw-2, Skip, Reverse, (Wild, Wild Draw-4)