Skyrim - This is an obvious answer from anyone that likes to play open world games. The freedom to wander through the vast open fields and snowy mountains, makes this game one of the best to play after a long day at work! It's easy to break away from all the quests and just roam the lands, and I always felt there were more places to find.
Far Cry 3 - Absolutely amazing game for all those times when you just want to run into a camp and kill everything in sight! Another gorgeous game with plenty to offer and plenty to explore.
GTA V - Because what's more fun than driving around with a bunch of friends and causing havoc? GTA has always been a favourite for me, although the map is limited, you just can't beat driving around with friends, having a laugh, and killing each other in the process.
Test Drive Unlimited - Hands down, the most fun I've ever had in an open world game. Sure, all the others are great, but you can't beat having a group of friends ( 8+ ), all in sports cars and bikes, taking over the roads. The ability to create an island long race that takes 40+ minutes to finish! The simpler times gameplay was more important than graphics. I remember spending days, even weeks, driving with one friend in particular, just trying to unlock all the roads and find all the dealerships. So much fun
So, that's my short list of the few games and I memories of that jump out at me. My all time favourite has to be Test Drive Unlimited, simply because of the online play. Games for me are always more fun when they can be shared with others.