Here's my top 5 from what I've owned:
1) 12" 1.5GHz PowerBook G4 - The last machine to carry the PowerBook name, and what an ending. Small, light, yet a full-featured PowerBook that runs Leopard like a champ. I still use mine daily in my classroom, and it serves me well.
2) Power Mac G5/2000DP - Dual G5s, air cooled, takes up to 8GB RAM. Toss an SSD in one and stand back! I love mine and use it for much of my PowerPC high-end software. Mine is actually my main PhotoShop machine, as I haven't had a need for newer versions of PS. It's powerful, expandable, and so, so beautiful. I still think that the cheese grater case is one of their best designs, and Apple agreed, keeping it around for the next decade through to the Mac Pro 5,1 that's on my desk.
3) 15" iMac G4/800 - The iMac G4 is hands-down the most beautiful machine that I think any computer company has ever created for the mass market. I have a near-pristene example in my kitchen, hooked up with the small crystal globe speakers. It serves me well keeping recipes and playing music while I cook. Hands-down gorgeous.
4) PowerBook Duo 2300c - I owned one of these for several years, and used it on and off for my writing. The ability to easily dock a notebook was truly revolutionary to me. I am still in love with the idea of having the exact same setup at my desk as I have out on the road, and the Duo allowed that to happen.
5) Power Macintosh 6500/225 - I had one of these Power Macs for many, many years. For a few years it was my primary Mac, then secondary, then that machine that I simply could not part with. I haven't used it regularly in years, and I can't remember the last time I booted it up, but it still retains its special place in storage and in my memories.