Tiger is obviously the best OS I have ever used. Really nice.
However, I found Panther to be perfectly fine. I found Panther to be a very solid OS with what you need, and then Tiger started to add things you needed, but did not know you needed. They ended up very big things though.
Of course, it also brought Intel to the Mac.
Leopard is going to add a few more things that we'll realize we needed, once we have them. Furthermore, it's going to polish the way we interact with our Mac and the way it looks.
Tiger should be remembered fondly as well as Panther. However, Tiger will have more of a legacy as there have been many more Macs shipped with Tiger, and Tiger supports still a good number of older systems. Some people will be running Tiger quite a long time.