Roku or Apple TV would be my top choices.
For Roku, it's their core business, they do a great job and I've found Roku TV integration much more user friendly than Android TV integration, of which I have both available but only use Roku.
Apple have a great interface, they support their hardware for a long time, if you already use Apple devices it's a no brainer and you can (currently) save $20 from the Apple refurbished store. My biggest gripe with the Apple devices is the slippery remote control which benefits from a (cheap on Amazon) silicone cover to stop it sliding between couch cushions!
A friend of mine swears by the nVidia Shield, though I believe it's getting quite long in the tooth as well as being one of the most expensive. It runs Android, as do many other no-name streaming boxes, so if that's your preferred environment it may be worth a look.