While i agree its a great game, saying its the best game in a decade is a pretty bold statement.
Bold statements on the intarwebs? Never.
In the last 10 years, the following games have come out.
1. Portal (if you dont like this game you are wrong and should feel bad)
Have it, love it, still play it regularly.
2. Starsiege and starsiege tribes. Both where awesome games (i liked starsiege better than tribes, and to its credit the old master server is STILL up and running and people are STILL playing it online.. just.. a dozen or a score or so is all thats left ;_;)
Played both quite a bit back in the day. I was a pretty effective artillery dude.
3. Dwarf fortress=P This is a great game once you figure out its complexities.
4. Xenogears. Whos initial japanese release was february 11th, 1998 and so falls within the boundaries of your statement.
Haven't played either of those.
There, 4 games from 4 different genres that are all great and awesome. Im not sure i would be able to say if this game is 'better' than they where. Im sure there are plenty of other great games, but these are the ones that have come to mind.
If you would shift your claim to the best game in its genre then i would be inclined to agree. But then thats like saying you are the most popular person on a deserted island.
I stand by my statement, because I'm making it from *my* perspective. RTS/TBS/4X games have always been my favorites. In the real world I play Starfleet Battles whenever I can find an opponent, and I play complex variants of the same game type in play by mail/play by email games.
Since I like the genre better, I like Sins better than any game I can think of since Dune II.
Now, my feelings won't be hurt if others fail to share my opinion.

"Best" and so forth are very subjective terms anyway and I'm not speaking for anyone but me here. If I were speaking as a game reviewer or something, I'd use different language.