My favourite one is 10.3 Panther with the mix of aqua and brushed metal. 10.4 Tiger is similar but it has a glossy top menu bar that didn’t age well in my opinion. 10.5 Leopard has the fancy cheesy 3D dock, transparent top menu bar, and the more modern gradients. It looked great at the time but gradients aren’t as cool as brushed metal and aqua.
Everything after is a bit boring.
bshacklett
Panther was peak OS X. Spotlight, in Tiger, was amazing, but Tiger’s performance was significantly worse.
holmium
If only someone could back-port Quick Look into Panther! I didn't realize how much I use that feature as I spammed the spacebar in these emulators.
al_borland
I'm pretty sure Quicksilver was around in the Panther era. It was great to have Spotlight built in, but Quicksilver was incredible.
Everything after is a bit boring.