Hmmmmm, tough question.
I enjoy ANY monochromatic light, though I suppose I do have some favorite W/L's.
In the red range , I still love the 632.8 HeNe line. Though, I may be influenced by the inherently high beam quality of HeNe lasers. I just love those gas laser beams.
In the green range, I do like 532nm, but I have to (once again) side with the HeNe line in this range (543nm). It's just more "green" in my view, and is more of a "lime green" when compared to 532. If you compare 532 and 543 side-by-side, the 532 will look almost blue-green in comparison. The 543 line is a "deeper" or "warmer" green and I just find it more appealing.
Now, my ALL-TIME favorite has to be 458 or 460nm. I do like 445 and 473, but right there in between them is 458, and to me, it's just the most beautiful sapphire blue that I can imagine. Sadly, I don't (yet) own a 458nm laser, (although I have seen a 458nm in operation), but I can dial up that wavelength on my monochromator, and it's just a gorgeous color. Absolutely beautiful, in my opinion. If you have any cobalt-blue glass in your house, then you have a close approximation of the color that I'm talking about. It's really nice.
BTW, a monochomator is a VERY nice bit of kit, if you can manage one. It's potential use is extremely valuable to any laser enthusiast.
-mega