Best Mac-compatible all-in one laser printer?

Helping you determine which one device is 'best' requires that you provide more information.
Home use, office use, or both?
How many pages do you (would you) scan in an 'average' month? 10, 100, 1000...
Are the scanned pages single sheets? Printed on both sides or only one? Bound in books?
How many pages do you print in an 'average' month? Printed on both sides or only one?
Text, graphics, photographs, mixed? (What percentage of each?)
Most common paper size(s) used? How often do you switch between paper sizes?
How many pages are in the 'average' document that you print?
Do you need different types of paper (color, quality, letterhead) for different print jobs?
How many pages do you send/receive via fax in an 'average' month?
Do you require color (laser) printing?
How much money do you want to spend on the printer?
How much are you prepared to spend on consumables (print drum, toner, or toner/cartridge?)

Many all-in-one laser printers that are/were well-supported by Leopard (OS X 10.5.x) are probably no longer available new.

Finally, are you planning to upgrade to an Intel Mac in the near future?

If you have a trusted salesperson at a retail office supply or computer store, they can help you sort through the options, prices, and determine what is best to meet your requirements.

HP LaserJet M2727 NF, still available from at least one national office-supply chain. Runs on Mac OS X from version 10.3.x and newer, and the PowerPC G3 and newer, as long as you have at least 512Mb RAM and 100Mb free hard disk space.

HOWEVER: It's not highly rated by HP customers (~2.5 out of 5 stars) apparently for lack of Windows 7 customers and other concerns.

If an inkjet more in your budget range: HP OfficeJet Pro8000W. PowerPC G4 or newer, & only needs 256Mb RAM & 100Mb free hard disk space. Rated 3.5 of 5 by HP customers.

We're happy with our Brother MFC 8860DN, which we use in a small office setup. It's plugged into our ethernet network and available to laptops over Airport with no problems. Scanner software (Brother Control Center) isn't the greatest but you can also set it up to scan directly to a computer from the unit itself. It has a duplex printer function (the 'D') which we use a lot; make sure you check out the features of the unit before you buy.