Beacause of the uncertainty of renewal from the networks, a lot of shows either end on a cliff hanger or with a wimper of a ending. Although one perculiar one is One Tree Hill, that show keep finishing and then keep getting renewed. It really could have ended in like Season 5 or 6 or something when Lucas and Peyton left but they renewed it without two main characters.
Veronica Mars has a ending to the season, not the series
Dead Like Me just stopped
Firefly just got cut short
Buffy has an ending like every season but the last one they just made it bigger
There are only a few that ends in a satisfactory way for me, one was Friends. 10 seasons is a good number and by the end they had pretty much tide up all the characters, Ross/Rachel together, Chandler/Monica together, Pheobe married and Joey going to Hollywood. Which is quite difficult as being a sitcom, they could in theory go on for yearsssssssssssss.
DS9 was also good, the war makes a tidy ending.
The West Wing ends with the President's last term if office which is fitting.