I'll start this off with the caveat that my palate is experienced if not educated. As with all things beer, YMMV.
I first tasted all six back to back in the order that I have them pictured. The head on all of them was white and about 1/8th of an inch thick. I didn't pour aggressively, but I kept these on the low end carbonation-wise to keep in line with some of the more common English styles of beers. I then ate a few oyster crackers and drank some water. Tasted them in the reverse order. Ate some more oyster crackers and drank some more water and tasted them in no particular order.
These were my initial impressions.
2-row
Bready. Very light. Very slightly sweet with a smooth light graininess. Very faint twang. Probably from S-04 yeast. Serviceable if a bit nondescript. It's amazing how you can just tell this is the base malt for a lot of commercial beers. It tastes "familiar."
Maris otter
Toasty on the nose. Much nuttier than 2-row. Slightly sweeter and more caramelly flavor. Toasty sweetness comes through. Toasted bread.
Golden promise.
Very mild aroma. Flavor is slightly sweet. Cracker sort of breadiness. Grainy. Sweetest of the six. Pleasant.
Pearl
Slight sweetness and caramel on the nose. Faint "green" almost vegetal aroma. Not unpleasant though. Medium caramel flavor with a smoother mouthfeel than the others. Sweetness carries through.
Optic
Very little nose. Slight oat aroma. Flavor leans towards the sweet grain side. Maybe a little rougher than the others. Almost grassy. I like it though.
Halcyon
Light nuttiness in the aroma. Grain is more present. Subdued caramel. Maybe a bit like the crusts from white bread. Really clean. Light nuttiness is nice. Very mild.
The three that stood out for me as the most distinct from each other were Maris Otter, 2-Row, and Optic. The Optic smelled like a clean barn (not barnyard though). Good horse feed. A definite rough graininess that I could see complimenting some styles very well. Maris Otter was definitely the most toasty of the 6 with some deeper more rich flavors. 2-row was, like I said, kind of "familiar" tasting. There's a reason it's the base malt for so many good beers. It's pleasant, unobtrusive, and doesn't assert itself in any particular way.
All that being said, however, I think my favorite was the Golden Promise. The residual sweetness resulted in a fuller mouthfeel without being too much. The cracker-like breadiness gave it a crisp flavor that balanced out the sweetness nicely. I still think there are some styles that would benefit from this kind of flavor more than others, but in the right place, this seems like a really nice malt.
After I feremnted these out, I did a little digging around about the yeast that I used, Safale-04, the Whitbread strain and noticed that some people mentioned a twanginess about the finished beer. All of these definitely have a slight twang. I'm hoping it clears up a bit with time, but that's one flavor that came across in all of the beers.
It was a fun experiment.
I've got four 6 packs of these beers left. I thought I'd throw it out that if there are any interested parties in the DC/Maryland area who would like to have a bit of a tasting party and experience these beers for themselves, I'd be happy to host a few people. Shoot me a PM.