I'll second the Nokians. I got about 90,000 km's out of mine and am now going to do winter/all season and the all season will be the Nokian WR G2's.
I will say the are not the greatest on ice, but not bad either, and better than all season. For snow, I found my stock all season Duellers were as good, maybe slightly better than my Nokian WR's, but that was the older tread design too. I found them to slip a bit in the snow, and was very disappointed in them for that.
For wet conditions (rain and even slush) they are fantastic. A huge step up over the Duellers which would start to hydroplane on me once I hit around 115 km/hr. With the Nokians I could go 130 km/hr easy and felt like I was on dry pavement.
For dry weather conditions they were fine. Not as good as summer tire would be but better than I thought they'd be.
On gravel roads, they didn't do bad their either. I think a more aggressive tread would do better there though (as it would in snow).
Now, as already mentioned, they are pricey though, very pricey. The reason I'm getting them for spring/summer/fall though is because sometimes we can get our worst driving conditions, especially ice, in spring/fall. Unfortunately in between snow/ice storms it can be 25C and I really don't want to chew up my winters, so I want a tire that can handle the odd snow/ice storm.
I also believe that the Good Year Triple Tread is an 'all weather' tire. Though I have no personal experience with them so I'll leave any comments on those to people who have/had them.