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It's a tie for me.

For me personally it's a dead heat tie between Tiger and Leopard. My first experience with OSX was installing Jaguar on my G3/300 tower. At the time to me it was amazing. A huge leap forward from os 9.2.2. Though in looking back at it now it was painfully slow. I chocked that up to living in Turkey at the time and being on dial-up that was hideously slow.
Panther on my Sawtooths and Digital Audio was meh, upgrading to Tiger after that was..WOW! It was my main os for the last 6 years-ish until I purchased a G5 with Leopard. Since then I have a macbook with SL on it. (Perfect os of that laptop). And 10.7 on my mini. Don't really like lion all that much, things are just a bit too different for my taste but I manage. 10.5 gets the most use these days on my road warrior.
 
Definitely Tiger. Spotlight was what really put the icing on the cake for me. Anyone else miss the blue end caps on the menubar in the Tiger DPs?
 
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