Definately Tiger... but Panther is a close second. I use dashboard often, but only for the calculator and the white pages (people) widget. Spotlight it nice, but it's not that great. I've gotta say that 10.4.6 is my favorite - it's much faster than 10.4-10.4.5.
10.3 was a great OS it was so much better then 10.2. Its a must upgrade to expose is great. Whenever I got to a windows machine I am mad because I can't switch windows as easy. 10.4 is still my most favorite though, but only because it has everything 10.3 has and more.