If you don't actually want a smart hub
As we said earlier, you don't have to drop extra cash on a device that serves only one purpose. There are smart speakers and other connected devices with all the components you need to unite everything in your home. Cheaper smart speakers like the Google Home Mini and the Amazon Echo Dot don't have the tech components necessary to act as a proper hub, but they can serve as a wrangler as long as you don't mind taking the time to set up trigger words and routines.
Amazon Echo Plus
If you want to build out your smart home but don't want to shell out too much money at first, the Amazon Echo Plus marries all the most essential features at an affordable price. This isn't a device to consider for a robust setup, though, since the Echo Plus is only outfitted with a Zigbee radio.
The $150 Echo Plus is a digital assistant, a speaker that can lead a raucous house party, and what I'd call half a smart home hub. But because the Echo Plus runs on Amazon's Alexa, its compatibility with third-party devices is nearly endless. Google Play Music and Apple Music aren't natively available through the Alexa platform, though they can be streamed over Bluetooth.
Amazon Echo Dot
If you don't need the Zigbee radio in the Echo Plus but you do want to be able to control everything by voice, the $50 Echo Dot makes for a fine alternative. All the smart home connectivity happens through the Alexa app, which supports voice commands and allows you to schedule routines for any linked devices and services. On the device itself, you'll find playback controls and an auxiliary port around back, in case you want to use it as a controller for an existing stereo system.
Still, the Echo Dot isn't a substitute for a full-fledged smart hub; it's really just a barebones smart speaker. But if you're simply planning on controlling a few WiFi connected lights or outlets in your home, this is all you need. Just make sure not to purchase anything that requires Zigbee, Z-Wave or some other proprietary connection to work.
The $129 Google Home speaker comes in three varieties: the standard Google Home, the Google Home Mini and the Google Home Max. Think of them as the Google Assistant in sizes small, medium and large, with each using the Google Home app to manage your devices. From there, you can set up shortcuts and routines, as well as identify rooms around the house. And since it's the Google Assistant that underpins this family of products, you'll also have access to Google's ecosystem, including Chromecast devices.
Like the majority of Amazon's Echo speakers, Google's smart speakers are not smart hubs, because they don't offer Zigbee or Z-Wave compatibility. If you want to add devices that connect that way, you'll have to build that bridge with an additional hub of some sort and then link it through the Google Home app.
If you're committed to the Apple way of life, you might consider the Apple HomePod ($349), which is both a hi-fi speaker and a smart hub of sorts. The HomePod has Siri as its digital assistant, which isn't as capable as Amazon's Alexa or the Google Assistant. (Heck, it's not even as capable as Siri on Macs or iOS devices.) Apple also uses a proprietary standard called HomeKit, which has increased its compatibility since the speaker hit the market. HomeKit works with devices from Belkin's Wemo line, Philips Hue series, IKEA, GE and Logitech, to name just a few examples. (Here's a complete list if you're wondering where to start.)
If you're not especially interested in the speaker, but you'd still like something with HomeKit support, the Apple TV 4K can also serve as a gateway device while streaming 4K content to your TV.