It depends what you call "easy"
do you know what you are doing? What do you expect from a modern OS? Do you have a nice internet connection or are you still stuck in 56krap land?
I have experiance with the following.
If you are like me and don't considure the "install" done untill I have all the hardware properly configured, all the services configured, and the OS properly hardened then WinXP is very hard to install.
Redhat 7.0 was ok I only had 56k (actually 32k)

at the time and got it with a linux administration book . And If you want a all the features that Linux can provide I would guess that mandrake would be easiest.
Openbsd was by far the easist to install if you know what you are doing. If you are not experiance enough to deal with the basics I would think it would suck very much.
Slackware 8.1 was a breeze, but if you are a gui man I would suspect you could get lost easily.
However If you don't care a wit about providing network services, or security, or money, or privacy then I would have to say that WinXP combined with a decent firewall/router (for security) I would have to say that WinXP is the king (especially if your a gamer). And thats definatly ok, too. I don't look down on people who only want to use a computer instead of learning about it. (that's what I want them to pay ME for. hehe)
I would suspect that for all of the above the Mac Os X would be the best compromise and would fill most of the requirements from above. To bad you qould have to give up some performance to get it (not as much as one would think though). I would love to get my hands on one of those laptops....
(the most horrible installation I ever did was to install Redhat 7.0 linux on a ancient Compaq Proliant 1000 server. It has a eisa bus (no pci means no/or marginal hardware detection) and a unsupported (at the time) embeded scsi array. NO IDE. I had almost no linux esperience at the time. Any windows (exept NT or 2000 server $$$$$$$$$) would never had gotten off of the ground. (ironicly DOS works well with it). hell in a 80 pound server. It took me 4 months to get it properly working. Now with modern linux it takes about 2 hours....) I considure that progress