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[sonaro]

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[ZRTMWA]

Technically greenish yellow is most visible. 555nm to be exact (small pic):

 

[charliebruce]

If it's a silly question, why ask it? The info can be found in stickies, but I'll help anyway.

Green is most easily-seen by human eyes, because sensitivity peaks at about 555nm, and drops of in a bell-curve from there, down to a low point at about 400 and 700nm. Violet light scatters (rayleigh scattering) most in air, meaning that a violet beam with the same energy and corresponding frequency (opposite side of the bell curve) of red beam, the violet would be more visible than the red. Of course it depends to some extent on the angles you view the beam at too. Personally, I find violet beams to be slightly more visible than reds of the same power at "normal" viewing angles (laser in hand), but eye sensitivity varies from person to person, and you might see otherwise. Yellow is much closer to 555nm than red, so mW for mW it is brighter, but most yellows don't reach beyond 20mW whereas reds can hit 200mW, so that difference is more than made up for in output power.

EDIT: Graph in above post shows the bell curve nicely, thanks ZRTMWA

 

[ossumguywill]

just so you know, 555nm is the confirmed highest sensitivity of brine shrimp. Humans are a bit off, I'm not sure we're 100% sure that 555nm is our most sensitive point.

will

 

[ZRTMWA]

No, it's 555nm for humans too:

Luminosity function - Wikipedia, the free encyclopedia

Wavelength of Maximum Human Visual Sensitivity