Geez, that's a tough one. In general, lumens are the best measure. Candlepower is measured in a globular pattern whereas lumens are measured at a target 1 from the source.
Watts aren't too bad to use as a general measure but not nearly as good as lumens. I could have an older 3 watt LED that puts out 50-60 lumens and put that up against a new Cree XR-E Q5 LED that can put out 240 lumens at the source.
That's because the technology of LED's is improving at a fast rate. LED efficiency measured in Lumenius(sp?) Flux. Three watts of
power is now brighter in the latest LED's compared to the older ones at the same power draw.
Compare it to vacuum cleaners. "My vacuum has an 11 amp motor" Mine has a 12 amp motor? The one with an 11 amps may be the better one due to the efficiency of the motor. Clear as a bell, huh?
Even the little 5mm LED'S have improved. The latest ones are 40% brighter at the same current. I have a flashlight in my pocket that uses the new Nichia DS and now the GS is out that's 10% brighter than the DS.
Link to Arc Flashlights
When you get into incandesint lights, watts are a little better to compare with but not always. I have a Streamlight Ultra-Stinger that is 10W and a Pelican Surge that is 7.5W. The stinger is brighter
at the source. If the Pelican had a tighter focus, it would
seem brighter to the eye.
My 3D Mag has a 20W bulb and a tight focus. It's what I call a barn burner.
As for headlamps, I will only use LED's. Way brighter, less heat and a longer battery life. A great headlamp is the Princeton Tec EOS. A 3 watter with 3 levels of brightness, 3AAA batteries on the front.
A rear battery, 4AA, is the Princeton Tec Apex. And it can be modded to twice as bright for $14. See this thread
Princeton Tech Apex Mod - Step by Step - CandlePowerForums
I really like my flashlight hobby.