I really like ImageOptim. It’s useful, easy and fast to use, and most of the time yields very good results.
Another great option to get your images as small as possible is Squoosh. It's not as fast, mostly because I always forget its name ;) I'm kidding, it’s super fast.
The only point I’d have is that it’s a little less convenient if you need to optimise a lot of images. You’ll have to optimise and download all of the images seperately. Depenind on what you ‘re after, the results with Squoosh might be even better than ImageOptim. As always, mileage may vary and I recommend you try them both to see what works best for yourself.