You're very spoilt for choice Rudi, and it depends on when you are thinking of going. My experience is fairly limited in terms of going there for the sake of photography, but I have been going up there all my life for family holidays. Personally, for beautiful scenery, I love the area around Crieff, and I know when I last went in May I saw nature in abundance - Black Grouse lecking, Hares, Red Deer, Ospreys, Short Eared Owls, Red Squirrels, Red Kites, etc. I've seen Eagles up the glens in the past, and there is a remote moor above Fowlis Wester where plenty of different bird types can be seen on the right day. Glen Quaich near Kenmore can be good, but I gather they are building a wind farm there, so that will change things
Having said what I've said, my parents have just been to Scotland for 2 weeks (Crieff area and Dumfries area), and in spite of my Dad having his binos glued to his eyes for much of the time, they saw very little of interest, disappointingly. Maybe wait until the Spring?
The other area that I love is around Boat of Garten. Again, cracking scenery, and the opportunity to see plenty of wildlife. Some of the forest and loch walks there are just beautiful, and of course the Cairngorms are fabulous too.
Just get North of the border and head in most directions and you can't go too far wrong. I'm sure others can tell you about their favourite places.
Good luck, and let us know where you decide to go, and of course, post the pics
Alex