We are far too negative on Crowder of late on these forums and too eager to show him the exit. With that said, of him, Bradley, and Smart, I'd prefer he goes.
First, that contract is valuable and should be able to fetch something valuable in a trade. Not sure about Bradley or Smart's expiring deals.
Second, beyond this season I'm not sure how much of a role Crowder has on this team. By the start of next year we'll have at Crowder's position: 1) one of the best SFs in the game in Hayward, 2) Jaylen Brown entering his 3rd season, and 3) Jayson Tatum entering his 2nd season. If you want to re-sign IT (and maybe one of Bradley or Smart) in the offseason, you can't have two lottery picks as your backup small forwards, plus another guy. Even though Crowder's cheap for the player that he is, that's not an efficient allocation of resources.
Third, Crowder is not big or physical enough to play heavy minutes at the 4, especially alongside an undersized 5 in Horford. Eventually I'd like to see Tatum put on some muscle and play that position, even if it's not full time.
Fourth, and this is the most important... Crowder is not the same defender that he was in his first couple years in Boston. I don't think it's related to effort or falling in love with his offense. It's probably more related to his ankle injuries. Who knows. All I can tell you is that his defensive reputation is better than his actual defensive prowess at this point.
Lastly... Crowder has some major "dog" in him. I couldn't care less that he's been salty on Twitter and takes everything personal. He's highly competitive, accountable, and by all accounts a team player. Every team needs Crowders. You'd definitely miss that attitude, but then again, you'd miss it even more if you lost Smart instead.