Best value on a game is proberbly Just Cause 2, i bought it on the internet awhile after release for about £5, then played the hell out of it racking up about seventy hours of gametime, i still go back to it for an odd hour or two aswell.
Worst title this is a difficult choice because i generaly only buy games i know i'll enjoy, i guess i'd have to say Savage, its an RTS combigned with First/Third person shooter/action gameplay in a team based Death match scenario, you have one person as commander for each team handling all the RTS elements and everyone else is a soldier, the game itself was actualy quite good but it was so full of people cheating and so on i gave up. I recall the moment that caused me todo so, i spent ages farming money and kills useing the sniper electron staff thingy to save up and upgrade my character to a behemoth with huge health pool and massive dammage basicly so we could attacked the enemy base, i took maybe three steps out of my own base only to be insta killed by somone with the basic weapons and no upgrades because he was cheating.