I don't have any game that I have ever been 100% boredom resistant to, mainly because I always push my luck and if I find a game I like, I play it whenever possible and don't stop until it becomes boring. Payday 2 is the game that has come close to being always fun recently. About a year ago it was a Free weekend thingy on Steam and I played it nonstop the whole weekend, probably put 50 hours out of a possible 72 into that game. I felt I was bored of it and felt I had made a smart financial choice because I now knew that I had experienced the game, gotten the jist of it, and would not need to purchase it. However a few months later I bought it because I realized I wasn't done and it is currently my second most played game on Steam.
My most played game of all time on Steam is the tricky example that I think might have been perfect to play forever without getting bored. I loved Team Fortress 2. I say loved because I hate it now, but I really loved "vanilla" TF2's balance and dynamics. I used to spend 5 hours every night playing 2fort and it never got boring. I had around 500 hours just as the Scout before they started updating it. I kept trying to enjoy it but the new weapons unbalanced the fine system I enjoyed. Ultimately, I never got bored of the game, I feel the game I loved was taken away from me and replaced with something far worse. I don't know if I would have ever gotten bored of basic TF2, I'll never know because that game no longer exists. That said, what I enjoyed about it was very tentative and probably wouldn't have continued to exist, I really liked playing on 24/7 2Fort servers with high player counts. Switching around between roles as one became tedious made it always interesting and when people actually cooperated and tried to take the intelligence it was very fun to take part in a team effort. However, without at least 8 or so people per team it was significantly less fun, so I doubt the game would have maintained that over the years.
Some games that I can often come back to and replay but do get bored of temporarily are FTL, Ratchet and Clank (2 & 3 on PS2), Counter-Strike, Bioshock 1, Hotline Miami, and Postal 2.