They all have one issue or another, honestly. I have them all, and for my money, the Roku Ultra is the best for most people.
I would say Apple TV, but that remote is terrible. Roku has the best feeling remote of any (IMO). I have a universal remote I use anyway (so I can't use Siri, but honestly I use the Roku 90% of the time). The Roku is also almost 50% the price of Apple TV.
I say all this, and we are an Apple house, all Apple computers, iPads, iPhones, we even have several homekit devices (which the Apple TV serves as a Hub for). I do appreciate Apple TV for it's better development possibilities (we use Mr MC for Plex, which can't be done on a Roku), so I understand why a geek would get more use out of an Apple TV.
But, again, for the average person, who just wants to stream on a TV, Roku is easiest.
I agree, for what you are asking, chromecast isn't really "stand alone". I tried a Fire Stick for a while, and it felt sluggish compared to AppleTV and Roku. It also crashed and hard reset several times (this was about a year ago), so I kinda gave up on it. I didn't care for the interface anyway.
I've never used a Shield, but they seem pretty awesome for the right person.