Both of these games are must-haves.
Borderlands: The Handsome Collection is the first free title that you'll get along with your PS Plus subscription which couldn't come at a more perfect time considering that Borderlands 3 launches in September. So during these slow months of Summer without much to play, you can get caught up on the Borderlands series.
The other game you'll get for free is Sonic Mania, which is my personal favorite Sonic game and truthfully, I also think it's the best Sonic game that exists. Anyone that's ever been a fan of a game of that series in the past should absolutely give this a download.