I prefer an all-natural, organic, artisanal, custom-blended chain lubricant made of crushed black slugs, hot tallow, and beeswax. Add some pine tar for wet conditions.
But more seriously, I've used many different brands of chain lube... in dry conditions I've not done any scientific tests,
but Rock n' Roll, Finish Line Dry, Dumonde Tech, Boeshield T9 all seem to work well in the dry and dusty conditions that predominate where I live. For wet conditions (read: what I put on my commuter's chain), I use Finish Line Wet because I bought a bottle of that a couple years ago and still have some left, it's thick and messy, but I like it because it holds up extremely well to being exposed to snow and rain. I hear really good things about the Rock and Roll Extreme (blue cap) lube for wet conditions too but haven't personally tried it.
When asked this question in person I tell my customers that it's less important which lube they choose, as long as they pick one and use it correctly (wipe chain, apply lube, remove excess) on a regular basis.
