I think Portal is Valve's best game.
While the Half-Life series is possibly more revolutionary, and I admire the design of the Left 4 Dead games, Portal is, to borrow a phrase from good old Trent Reznor's most popular song, closer to God. It is one of the best-loved puzzle games of modern times, a superbly, tightly designed experience that started life as Narbacular Drop, a student game from a bunch of very smart young people at DigiPen.
You have probably played Portal. But if you haven't, it's a first person puzzle game that plays deviously with physics and a touch of non-Euclidean geometry, tasting the player with solving a series of devious test chambers using a portal gun that punches holes in space-time. It has a near-perfect learning curve, and a memorable frenemy-adversary in GLaADOS, a sarcastic robot lady who chides you on your performance.
I think everyone was sick of The Cake is a Lie memes within about a week of the game's release, but even playing today, Portal holds up beautifully in terms of its level design, gently bananas storytelling, and sheer inventive vision. I will never get sick of watching players and speedrunners find new ways to break the hell out of the game (like this run without using portals) and exploit the cleverly arranged geometry in every which way.
One of the coolest things about Portal is that it makes you feel smart. Not just smart, but like a genius who can take nothing more than a companion cube or two and rustle up a brilliant solution to the designers at Valve's worst and most devious concoctions. That's all part of the plan, of course (and I'd be willing to bet on some iteration of the team's player experience goals including "make players feel smart"), with stages that grow steadily in complexity without appearing monstrous, and require you to apply all of your knowledge at a just-right pace.
I am in awe of design that can accomplish this, challenging the player without frustrating them. By the end of Portal, in just 5-or-so-hours, you are a master of its brand of physics, making wild leaps, accelerating out of portals, punching impossible holes in eroding walls and pipes and machinery that you were never "meant" to see in the game's fiction.
There was a (very good!) sequel in 2011, and like most Valve single-player games, nary a peep after then. Is this sad? Yes. But nothing will mar the fact that Portal, as a piece of game design, is the highlight of Valve's impressive ouevre.