In a word, Mull. 
I can't think of many places that could beat it. We were there for just a few days at half term and saw over 60 species of bird (and heard a few more) - eagles (gold and sea), puffins, lots of other sea birds, plus otter, seals, deer, cetaceans (porpoises and a minke whale - often they get dolphins too). The Top Trumps seems to have faded away, I could have cleaned up there!
So now we're debating whether to go back next year or whether there's anywhere else that could compete. We've not really been to Scotland much, wondering about Skye....