Best, in my opinion, would be the Myst series. It's pretty much our universe, but you're able to write (if you have mastered the Art of writing) new worlds seemingly* into existence and travel there through the pages of book itself. Could lead to some very interesting journeys and experiences.

Worst. Well, I'm going with the popular opinion and will say Dark Souls. There's a chance that, when you die, you won't actually stay dead. You'll wake up as an undead, and will probably be brought to an undead asylum to wait out the rest of your days, slowly going insane and hollow. Or you can try to make your pilgrimage to Lordran, but you'll probably going to run into some issues unless you are very strong in body, mind, and will. You'll see lots of good friends lose hope and turn hollow, or turn on you and stab you in the back. You can try to die over and over as much as you like, possibly looking for some way out of this horrible existence, but you never really can die. You just eventually give up all hope and fall into a heap on the floor or next to a wall in some old, abandoned ruins somewhere. Still technically not dead, just immobilized by your hopelessness after having given up on life (At least, that's how I see it). Or maybe you'll just go insane and live the rest of your undead life trying and stop some of those pesky undead hero wannabees from advancing. Who do thy think they are, trying to act the hero? Somebody should put them in their place!

Basically, Myst is awesome, and Dark Souls, while it might be cool for a bit, eventually leads to despair, sadness, and possible insanity...unless you're a Sunbro. Then you'll always at least have Solaire by your side.

*Seemingly meaning some people believe you actually create the worlds you write, thus making you somewhat like a god, and others believe the world was already there and you have just opened up a link to them through the books.