I voted Tiger, but I'm also ver, very happy with Leopard. however, Tiger is what brought me
back to using Macs.
I say that because I had used Macs in high school, back when system 7 was first coming out (though we had machines using 6.0.4). Back then I liked Macs, they were decent machines, and I wished I had one of my own.
Unfortunately, it seemed like that waned as the System matured more. OS 8 seemed okay, but not
quite as okay. OS 8.6 seemed outright creaky, unreliable and slow to me. By that time, I had lost faith in Macs, I must say. To me the interface was dated and frustrating to use.
My next encounter with a Mac would be with the release of OS X 10.2, which made me realize that Macs had potential again. The UNIX-like interface really caught my eye and the interface seemed new and modern again. Still, I felt a little suspicious based on my past experiences, and only used Macs sparingly. By 10.3, I admitted I liked OS X, but couldn't yet justify buying a Mac of my own. But I liked using the one we had at work at the time, and found any excuse possible to use it often.
Then finally, Vista came out, and well, that was it for me and Microsoft Windows.

i used my tax refund in '07 to buy myself a MacBook Pro, and well, the rest is history.
Interestingly, it took several OS releases for me to get disillusioned with Macs back in the bad ole' days, but only one OS upgrade and a couple days for me to throw in the towel with Windows. Vista is/was
that bad.