Well, let's see...
Half-Life - Valve will probably never finish the series, but the two and a half games (plus the HL1 expansions) that are out there are some of the best games of all-time.
Portal - The original was an instant classic and the sequel was the closest we'll probably ever come to a Pixar movie in game form. Easily the funniest game I've ever played.
Monkey Island - You can discount the Telltale series (which happened before they found their niche), but the first three games are absolute classics. The third is my favorite adventure game ever, and is second only to Portal 2 in video game comedy writing (imo).
Mario Bros. - I'd include at least two of these games on my top ten all time list. I don't really play them any more, but Mario 3, World, and 64 have some incredible replay value. I can still load them up to this day and have a great time.
Legend of Zelda - Pretty much the same as the Mario series. Ocarina of Time is my personal choice for best game ever made, and Link to the Past is brilliant as well. Even the weaker entries in the series are fantastic. Note that Wind Waker was the last one I played to completion.
I could probably think of more if I kicked back and really thought about it. There are also some borderline ones, like Elder Scrolls. I probably sank more hours into Morrowind back when I was younger than the entire Mass Effect trilogy combined.
And if multi-player games count, the Team Fortress games are an easy one for me. I've sank over 3000 hours into TF2 since it came out, and that's just on the PC version. I played the 360 version for at least a year or so before I got a newer PC and was able to properly run the Steam version. I also played TFC (the Half-Life mod) back in high school and early college.