Some tire/rims seal better than others. It doesn't take much to lose air quickly regardless the sealant. I agree - flush out and change the sealant at least 2x/yr.
As for best tire - this is the Holy Grail. And as long choices exist, people will choose (and buy). With that being said - the laws of physics are pretty applicable.
1) Increases in wall thickness increase resistance to cuts and also increase dampening and compliance (i,e, make it lose energy)
2) Increases in dimensions increase the frontal area and increase wind resistance. (i.e. make it slower)
The best all around tire is the one you can ride every day. Because, lets face it, unless you're racing, who cares? The stop light is still longer than your PB.
My take -
Cheap tires actually last longer because they use higher vulcanized, molded treads. Look at mama charis. Some of these tires are going on 20yrs and still kicking!
Bigger tires last longer because there is less penetration dispersion. Same thing as mamachari. Get bigger tires and the stables and junk in the road will affect it less.
Best in rain generally means nothing. Because a) it doesn't rain all that often, and b) when it does, not many people actually ride.
Tire pressure means more to the ride quality of the bike (and tires) than anything else. Learn to adjust your tire pressure to deliver the correct ride quality YOU want.
With that being said -
Conti Gatorskins - like dragging a gardenhose through mud
Conti GPS4000 - fast on the straights, crack on the corners
Schwalbe Ones - smooth until you turn - then mush out faster than Mickey D fries
Panaracer A - dull, boring, like you are riding with a hangover
Panaracer Closer - Remember VW rabbits with broken struts?
IRC - We used to call these IRC Roadskates because they were certain death on any pavement condition. Exception is the new Aspite - I dont't have anything negative to say about them. Yet.
Challenge - remember the good ol days? Yeah, me too, and tires only lasted a week or so. Now with global warming and ozone conditions approaching a post nuclear test ground zero - they last even less.
Veloflex - It may not be the fastest tire for the day, but it will never be the wrong one.