It's kind of funny to me that I have so many liquid highlighters when I reach for my powder highlighters instead 95% of the time, but before I start eliminating the less-loved ones from my collection, I thought I should do a swatch and comparison post for you. You will definitely want to view the pictures full size to see the shade and textural nuances between different products. I have intentionally swatched these thickly to clearly show their differences; all can be more sheered out on the skin than how you see them applied below.
Outdoors (with my HappyLight supplementing the almost-nonexistent sunlight):
Indoors with flash:
Same swatches, closer-up to better show texture:
Becca Shimmering Skin Perfector in Opal, a pale bronze
Becca Shimmering Skin Perfector in Pearl, a natural white (this blends into the skin very well - better than you would think from the swatch)
Benefit Girl Meets Pearl, a medium-pale rose gold
Clarins Instant Light Complexion Perfector, a subtle pale pink
Guerlain Meteorites Primer, clear with sparse sparkles
Illamasqua Illuminator in Poised, a medium-pale candy pink and the frostiest of the bunch (this is the last one in the third image)
MAC Strobe Cream, a subtle, iridescent pink (similar to the Clarins one but slightly more difficult to blend)
NARS Illuminator in Copacabana, a pale pink (another one of the most visible and least subtle ones here)
Rouge Bunny Rouge Sea of Clouds, a vivid white (more noticeable than Becca Pearl)
Rouge Bunny Rouge Sea of Tranquility, a pale pink (more noticeable than the Clarins or MAC ones)
Yves Saint Laurent Dare to Glow in Naughty Pink, a pale pink (more delicate than the other pinks; has the effect of being infused with sparkles rather than a sheer frost)
I believe the Illamasqua one would be best for dark NW skin tones, but not many other complexions.
If you really like your highlighter to show and provide clearly visible illumination to your face, the most obvious ones on the skin after blending are NARS Copacabana, Benefit Girl Meets Pearl, and YSL Naughty Pink.
The most subtle ones that you could easily wear all over your face are Clarins Instant Light and MAC Strobe Cream.
The ones that I would recommend as more of all-purpose products (excellent worn alone as highlight or blended in with moisturizer/foundation and applied all over) are the Becca SSPs and the Rouge Bunny Rouge highlighters.
I think it was a mistake to include the Guerlain primer in this set of swatches, because I can see now that it is an entirely different animal compared to the others! I believe it is meant to be worn under foundation or alone to add a glow to the skin and a bit of sparkle.