Have to agree, Duck Game is a blast playing with friends in the couch. The Jackbox Party Pack games are fantastic in that context, too.
The number #1 game that hasn't and isn't getting enough attention is in my opinion Grand Theft Auto 2, specifically the multiplayer mode. I don't know if the popularity of its successors overshadowed it or what happened because it's soooo fun to play online, even today. It's free and with GTA2 Game Hunter it's not hard to find a game if you have a little patience. Tiny Town is my favourite map
Rainbow Six 3 is underrated as a LAN game. It's FUCKING difficult and unforgiving, and that's what makes it good. Trying for two hours to get more than a few meters into a bank at the beginning of a map with your friends...
As for single player games.... Broken Sword: The Shadow of the Templars, an old point-and-click action adventure game. It was very well received at the time of its release but now is largely forgotten. It has aged very well. The characters and dialogue are hilarious and loveable. I can't recommend the "Director's Cut" version. Go for the original version if you want to try it out.