My favourite is arch, for several reasons:
1. It is stable and bug-free
2. It has the greatest flexibility of any other distro I've seen
3. Pacman is the fastest package manager, ever.
Ubuntu is good, though it's a little too Apple-like sometimes in how it's run. PCLinuxOS is good though it reminds me a lot of Windows due to the KDE usage and its look and feel. Puppy is incredible for its speed. Bodhi just looks amazing.
Each distro has it's ups and downs so you just have to find what decreases your world suck the most. Ubuntu does it for me, for now.