You mean just the current machines? Well, right now I have to say the MacBook is the most attractive machine. It leaves very little to be desired. My only complaint with the machine is that it offers a little too much I'd rather do without bluetooth and the iSight, personally. It's got about as much processing power as any of their Intel machines, and it can support multiple displays, so it can really replace your desktop. I've never been able to say that about a relatively-cheap laptop before. Actually, this is the first laptop I've ever seriously considered buying (although I won't, since my G4 Mac Mini will last me until the current MacBook is obsolete, and I am not a slave to Upgrade Fever).
Honestly, I'd feel silly buying any of the other machines at this point.
If we're talking all-time favorite, then I'd have to go with either the original Bondi Blue iMac or the Ruby iMac DV+ (which served me well for about 3.5 years, btw). The original iMac was just a breakthrough. The Ruby model was my personal favorite looks-wise, and I also really appreciated its generation's lack of any fans (i.e., quietness). It was the perfect example of no-compromise style, which I'm sorry to say Apple seems to have abandoned; their current machines look boring and are too small for their own good, heat-wise. I'd like to see Apple go back to their old design philosophy and put more consideration into things like noise, which affect the user experience much more than a little extra speed or a smaller footprint.