I'm not really an expert in games, because after I found Galactic Civilizations 2 Dark Avatar I fased out other games. Most computer games I seen were in the last decade, but not made then.
Probably the worst game I've seen was Spore so much wasted potential in the game.
I'm going to have to say the worst game in the last decade is explorer craft it keeps crashing, and Everytime it crashes I lose its stuff.
Now a game that went from my favorite to my worst was Civilization. In four you found a mix for the perfect game, but in five on up they got rid of manually managing citizens, and allocating spending. This ruined the game for me. Civilization 5 and 6 are on any list as far as the worst games of the decade for me.
It's easier to come up with the worst for me I dont play a lot.
Now as far as the best it would be the game I can't get away from. First I like turn based you get points for not dumping that. I played the ones they call the best Master of Orion 1 and 2. Too bad I didn't play computer games back it the day. It would have been nice to see how galactic civilizations rank against them. By the time I played these I couldn't do it I was already playing Twilight of the Armor, and it is to hard to go backwards for me. Now because, you do more than any other game I have more to do it would be galactic civilizations 3, even the base game 1.7 or better is the best game of the last decade for me.