This is not a simple question. For a detailed discussion of some of the issues, read this article: http://www.electro-optical.com/whitepapers/candela.htm

Also, look at the chart at the end of the article comparing typical levels of luminence, because it shows how bad the human eye is at comparing brightness. Would you believe that bright sunlight is 1000 times brighter than typical residential illumination?

In short, the usefulness of a light source is determined by three things: how much of the electrical energy consumed is converted to light, how that light is dispersed, and how well the human eye processes the color produced.

If two lights are producing the same amount of light, but one is a floodlight and the other is a spot, the spot will appear brighter. If two lights are producing the same amount of light and one is red and the other white, the white will appear brighter.

Comparing lights is not just hard for cyclists, it's hard for everyone, and there is not a good number to use for rating them. Sometimes lights are rated in lumens, which is total light produced, irrespective of color or beam angle. As an analogy, think of a sprinkler rated in gallons per minute. This measure is only really useful for comparisons if a light emits equally in all directions, and is not normally used for lights with reflectors. Lights with reflectors are usually rated in "center beam candlepower" which is adjusted for color and measures the brightness of the brightest part of the beam. The problem with that as a measure is that it says nothing about the rest of the beam. A halogen beam with a flood pattern has about one-sixth the center beam candlepower of one with the same wattage and a spot beam.

Manufacturers seem reluctant to publish candlepower figures, but here are some samples I got from googling:

A 1W luxeon will give you about 200-400 center beam candlepower.

The Topeak Moon Shine HID is rated at 10w, 1300 candlepower.

A Sylvania 35W MR-16 halogen bulb is rated at 11,000 candlepower for a narrow (8-degree) spot and 1400 candlepower for a flood (40-degrees).

(I love this quote from the article: "At the threshold of vision the dark adapted observer can see a flash if it contains on average 90 photons at the cornea or 9 at the retina. This is equivalent to a candle at 30 miles on a clear night.")