We've been doing alot of tire swapping and riding under many conditions. Subjectively --
Conti GPS4000 II - Fast, harsh, unforgiving, reliable
Vittoria Graphene - Fast, smoother, agile, fragile
Specialized Turbo Cotton - Very fast, tactile, agile, fragile
Challenge Citerium - Fast, supple, 'tubular' , medium fragile
Schwalbe One - Fast, agile, some road feedback, solid
Panaracer EVO - Medium, OK, rougher, durable
IRC RBCC - Medium, OK, rougher, durable
Vittoria CX TLR - Insane fast
Rubber on the road counts:
23C - vertically non-compliant, best for high top end TT with skilled, smooth rider. Most riders today have forgotten the word 'souplesse' much less practice it. You need to be dead smooth to ride Pro 22.5mm tires fast. When you can - they are very fast.
25C - more compliance, more loss. more forgiving. a great choice for all around riding and newer tread formulas yield faster tires with lower Crr than previous. If you want to choice a tire - get a 25C and it will probably by 95% perfect.
28C - yet more compliance, top end drops a bit - but midrange is svelte. Long rides are comfortable more than you can imagine. Take on a little off-piste as well. For touring, training, commuting, etc - my 'go to' tire on modern bike is 28C
This is a loaded gun. Road surfaces and conditions vary widely. If there's only one thing to go on - that's width. Bigger is always better. No matter what. Stuff the biggest tire you can on your bike. Thats why we are seeing more and more 650 builds with balloon tires. More air and more rubber = more compliance = better suspension in rough surface.
Technical treads. IMO these are mainly marketing gimmicks. The various passes of Washington State (I've ridden most of them, mind you) , etc. Yeah, it's fun to mess around with rubber durometers, siping, etc - but I doubt it seriously makes any difference unless your name is Jeremy Powers. If you choose a tire for it's tread shape - then get something that esthetically pleases you. Because after 50mi or so on gravel, that's all that counts.
Exceptions - 1000km in the grass. 500km on mud, snow and ice only. Yeah - then find a tread / rubber that is optimized for these specific conditions.
@davidglitt while I have been a somewhat fan of gatorskins - its really only because they lasted the longest on my fixed gear bike. Mainly for skidding. You can skid a gatorskin down past 2 or 3 layers of tread and it still keeps going. Besides that, it's about as fun to ride as a glued together garden hose. Cornering is horrible, grip is so-so , ride quality is , well, garden hosey and aside from the sheer strength of the main tread, it suffered sidewall puncs and bites no better or worse than anything else I rode.
Tim's Top Picks:
Schwalbe S-One: 30mm goodness of fun and fast on and off road. If I was riding the Arenburg Pave - this is it. I still hold the KOM on the Potato Patch riding these tires.
Schwlalbe P-One: 25mm fast, reliable , raceable. Throw away your tubulars and just get these and be done with it. If you like a bit more suppleness or heavier loading, then grab the 28's
IRC Xguard 28: IRC hit their sweet spot with this tire. Darn reliable , yet decently supple. Rides fast but not as harsh as many tubeless. I spec these on all our Outlands and Vasara 'all road' bikes as well as our top end touring bikes.
IRC Serac Sand: The Poor Man's Schwalbe 30. At about half the price and very similar. Plus a bit bigger makes it one of my favorite Rindo Masher tires.