There are some great suggestions here already. I'll throw out some of my favorites that are all relatively inexpensive, easy to find, and easy to learn:

The Mind
Incredibly easy to learn and teach. I don't think I've ever played just once game because everyone always wants to play again immediately.

The Crew
Great cooperative trick taking game. There's a reason it won a bunch of awards, and the sequel is even better. Can't go wrong with either. Regular filler game for our game nights.

Just One
A party game where you have to guess a word. Everyone is the clue giver except one person (rotates every word) with the catch that all clues are one word, written in secret, and if two are the same then they get hidden and can't be used. We have gotten a ton of mileage out of this one. Lots of fun, 10 second teach, good for a group of 5+

Gloomhaven: Jaws of the Lion
It's admittedly a little complex, the only game on my list I wouldn't describe as easy to learn (unless you love tactical rpgs, in which case it's not bad), but there are great online tutorials you can watch on YouTube. It has a video game feel. You're 2-4 heroes fighting against the bad guys I a combat map that comes in book form. You can find consistently for $25 or less in the US, sales just keep popping up. Without question the best starting point for the Gloomhaven universe.