It's very, very big question.
Everybody can say completely different answer.
My personal choice for the last few years is Debian, that's why Sparky is based on it.
I like *buntu LTS (Lubuntu, Xubuntu), Mint based on *buntu LTS, openSUSE, PCLinuxOS too (and VectorLinux and Zenwalk before).
Puppy is different group of OS for me, best for live usage, not for installing on a hard drive.