Season 5 really is a good place to quit the show. There are still good episodes after that. But season 5 completes a massive arc in a very satisfying way and it'd honestly be the perfect series finale except for the last 5 seconds.

Either way though, I always recommend the S10E05 episode "Fan Fiction" You can watch it right after the S5 finale as it doesn't have much to do with anything that happened after that, and it's a fun meta episode as Sam and Dean find a school putting on a musical play based on their lives.

(Well, technically I guess it does have spoilers for the S6 premiere as it lets you know neither Sam or Dean are dead...but considering the show has 15 seasons that's not really a big giveaway)