Gratz DJIndy,
You've won this the March Best Forum Post Contest. This is a great question as I'm sure most if not all people have a game or a game series that they will never get bored of. There are a few series that I never get bored of and when they release sequels to them I always pick them up. Street Fighter and the Resident Evil series I never tire of (I am still desperately hoping they release a new Resident Evil game that features Jill Valentine as the main character, I love me some Jill Valentine).
But if you are talking about a single game that I still play fairly consistently, it's definitely Rock Band 4. Really, it doesn't matter if it's Rock Band 1,2,3 or 4. It's the gameplay aspect that I love. Rock Band 4 just has all the other songs importable. A buddy of mine has a gaming/anime/movie party every month (anywhere from 15 to 30 people show up) and I always bring Rock Band over there and we play until 4,5,6, even sometimes until 7 am (party starts around 8 pm the previous day). It's a GREAT party game. I always end up driving home bleary eyed and a little drunk in the morning

.
Even though we all suck at singing (well, a couple of the guys are decent), we still have a blast singing, playing drums and guitar (I mostly play the drums and sing). I've been told I can sing Imagine decently, but I'm a mess with most other songs (people have told me on multiple occasions that I look a lot like John Lennon so I specifically practice singing that song the most

).
I always check every week for new songs to download and have downloaded over 250 songs over the years (that's a lot of money I've invested in the game, a little here a little there adds up

). I even bought the Ion drum set (a lower end electronic drum set) with the electronic brain and have it set up in my basement, connected to a guitar amp in addition to the PS4 so that when you play along with the game, you hear the actual electronic drum sounds through the amp (it basically sounds like you are playing a real drum set while you are playing the game).
So anyway, yeah, definitely Rock Band 4, I'll NEVER get tired of that game and gratz again on your win DJIndy.
-Admin-