Here are some more good puzzle games that are not Portal/Portal 2:
- Colossal Cave
- The Entropy Centre
- Hourglass
- Myst
- Obduction
- Pneuma: Breath of Life
- Quantum Conundrum (one of the original Portal developers was attached to this project)
- Quern: Undying Thoughts
- Riven (this is the sequel to Myst, but unlike Myst, it has never been remade since it first came out back in 1997, so you’ll have to excuse the old graphics and gameplay)
- The Talos Principle (the sequel is coming out soon)
- The Turing Test
- The Witness
If you can only play one of the above, then as the other commentors have said, make it The Witness. The recent remake of Myst is also very good.
No, because none of the above have multiplayer.