Depends on what you like.

Some of the most replayable single player games I have played so far are...

San Andreas obviously is one of the best driving / flight / action-shooter games there is. The language is a bit rough and the story is annoying. But its openended nature makes it highly replayable.

Tony Hawk: yes there are alot of tony hawk games but get the newest one and it's sure to be good / if you like skateboard games. These are also openended somewhat because your free to skate around untill you want to pick up a mission. Also they come with online play and level editors.

Mercenaries like Andreasis another openended game that focuses on war. Its really fun lots ofvehicalsand really cool weapons. Alot of cool missions here and the graphics are excellent. The driving is a bit clonky but the combat is great.

Phantasy Star Universe is a game that isprobably betterfor kids than the others I listed and fun for everyone. The story mode is like being a character in a anime movie. The games action is somewhat repetative however the story isn't it. Extra mode alows you to make your own team and play the game as much as you like with rpg elements this makes it somewhat openended.

Last but not least Ilikethe Robotech games Battlecry and Invasion.They aren't openended and theydon't havethe best graphics howeverthe game play is fun and they have a arcade/fps blend that I find unique. Battlecry was the fist to come out and I still enjoy playing it they have a nice menu that lets you select any mission you have allready played and play it again with records and medals ect.

A few more mentionables here (might be short or repetative)...

MX vs ATV unleashed

Gun

Deer Hunter