Team Fortress 2 in my opinion, is one of the best games on steam, and here's why. Team Fortress 2 is a great First Person Shooter with 9 unique classes to play as. It has a matchmaking system that includes Casual and Competitive (but no one plays competitive). There is Mann vs Machine which is 6 players vs NPC robots but you would have to pay for a ticket or get one in a Holiday Stocking or other ways, or you can play Boot Camp which is MvM with no loot and it's free. You can play Basic Training mode but that isn't exactly the best. There are also bots you can play against which is great for learning the classes and it's A LOT better than the training. The community servers are a great substitute for Matchmaking since it has tons of different game modes such as Saxton Hale (boss fight), Randomizer (switches different weapons with different classes), Class wars (1 team of the same class vs the other team with the same class), and many others. There's something bad in everything good and something good in everything bad. This game doesn't get updated as often anymore but if you are willing to take that chance than I say go for it.
Overall my ratings for this game is really recommended 9.5/10.