If you're just thinking of a small home server, then Linux is probably the lowest maintenance, and relatively easy to set up.
Samba is a piece of cake if you use SWAT, or it's a not too hard to just modify the cfg files. (SWAT is a web tool for editing your samba configuration)
I've got an old Dell server that I bought back when the 120GB hard drives first came out running my home file server (also have a small ventrillo server running, and in the past, used it as a router, and have hosted games, database, and web on it before too)
I run slackware, but that's just because it's what I learned with ....
Linux is great as long as
1,) figure out exactly what you want to backup and how you want to plan it ... Stuff like tape drives, etc ... pretty much are all supported in Linux, but if you want to use some other method, make sure you have an idea how you wanna do it before you decide.
2.) does your server need wireless access? I've gotten wireless working on some machines, but not all. If you want wireless, check to see if whatever wireless hardware you plan to use is easily set up in Linux
3.) What kinda hardware are you running on your server? Chances are this is probably a moot point, as you can pretty much run a 486 with 64mb of ram and have a file server .... though if you want to install X and all kindo of other crap, and you want a "fancy" distribution like Ubuntu or something ... you'll probably want at least 1GB of ram and a P4. (my server has only 384mb of ram, one reason why I'm sticking with Slack)
That said ... I'd also consider OpenBSD, but maybe only because I'm a bit masochistic at times.
Also, Windows should also make for an acceptible server... however, you might need to hook a monitor/keyboard/mouse up to it every now and again ... and it may take a bit more maintenance .... (still should not be too big a PITA though) ....
I don't see there really being too many "bad" options here ....