The big 3 are FlightAware, FlightRadar24, and ADSBExchange. While some of them do use official data, all 3 of them also rely upon their own networks, mostly of user-contributed data. As such, more populated areas are more likely to have more receivers. Additionally, each provider's network may have different users feeding them. FlightAware shows their feeder locations here: https://www.flightaware.com/adsb/coverage/ If I had to guess, I'd say FlightAware has the most # of people feeding it, FR24 second, ADSBExchange a distant 3rd.

Users will commonly use a cheap Raspberry Pi computer with a USB dongle (RTL-SDR) radio, and a cheap antenna. I have a couple of these feeder setups myself and I feed all 3 networks. I've attached an image of a sample station below. In exchange for you feeing them data, all of these will give you their top-tier service free. Considering FlightAware's top tier is $99/mo, spending $100 once for a feeder seems like a good deal to me.

As far as who is the most accurate, that's up for debate. I've seen FlightAware used by airlines, charter operators, and FBOs. I've never seen FlightRadar24 in the wild.