Wow, This is actually an impossible question for video game collectors or elite gamers. You do realize that some people play way to many games to name one worst or best game? lol. But i'll try... (Note that these are the first games that pop to mind)

Worst =

Halo (Online)
Call of duty (Online)

I say this because of the majority of players. While I don't mind the black sheep/Asshole of the family and can easily ignore the little annoying voice, I cannot tolerate the entire family being an asshole. Playing mainstream shooters online is like being the only reasonable adult sitting in a class full of unbathed teenage punks (No offence to teenages, I'm only making a reference).

Best =

Currently -
The Monster Hunter series
Dark souls/Demon souls

While I am a pretty big fan of most RPGS, Both The MH and Soul series take everything to a whole new level of creativity and complexity. Monster hunter for me is a great experience that lasts a very long time. It's extremely flexible and full of things to do. Also the online community (Although Fairly small in the US) is very likable and chatty. The soul series has got to be the most frustrating and controller smashing game i've played in years. This alone earns it a big spot in my top 10. Plus its very creative and customizable in pretty cool ways. Also the dark vibe it gives off is simple awesome!

So there you have it, Off the top of my head.

Top 5 best:
1 Spyro: Ripto's Rage (Called Spyro: Gateway to Glimmer in Europe) and Crash Bandicoot: Warped
2 Kingdom Hearts II. The best of the games in the series that I've played.
3 Skyrim. It's the only game in the Elder Scrolls series I've played, but I still like it a lot.
4 Portal and Portal 2. Hey, there can be a tie, and they can also be part of the same series
5 The Jak and Daxter and Ratchet and Clank series tie for this.
Top 6 worst, cause screw the rules, I'm a pony:
1 Shadow of the Colossus. Seriously, the game looks like someone wiped their ass with it, and the game doesn't even tell you where you have to go at all, and all that's there is a huge empty landscape.
2 E.T. for the Atari 2600. No explanation necessary
3 Spyro: Enter the Dragonfly. The game wasn't even 5% finished!
4 Any game for consoles that was also for GameBoy. Seriously, most of the time the GBA version has almost NOTHING from the console versions. The Namco Museum games are the exception.
5 Almost any GameBoy game that's part of a series that's only on consoles. An example would be Spyro: Season of Ice. Kingdom Hearts: Chain of Memories is the ONLY exception to this.
6 . Spyro: A Hero's Tail. Swapping the

Worst Game I Played: Stuntman

Best Game I played: Civilization Series