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Before 7 years ago, I'd have said RedHat.
7 - 2 years ago, I'd have said Ubuntu.
Now, I'd say Debian.
Squeeze is a nice, stable system, and with the squeeze-backports you get some up-to-date packages that are really nice to have -- and Wheezy is looking quite nice as it matures into stable. I'm looking forward to it.
RedHat lost me when they forked for Fedora, and Ubuntu 'just worked' so much better on my Dell laptop at the time.
Ubuntu stuck with me until they started throwing away all the productive elements of the GUI in favor of using the masses as guinea pigs. Their sever product is totally fine, but I refuse to dog-food on that desktop environment. So down the stack to Debian I went, and I'm -very- happy here.
Gosh do I wish Haiku were a -tad- further along...