While there's not necessarily a simple answer to this and each platform has its pros and cons, on balance, LG's webOS is probably the best TV operating system right now. Its selection of streaming services is more or less flawless, and every app has been optimised to work at its best. It's quick and intuitive, too.
Samsung's Tizen platform used to be the best but it's become increasingly convoluted and unintuitive in recent years. Hisense's VIDAA operating system, on the other hand, is getting better all the time and should now be seen as a positive feature.
Unlike the above, Google TV (and the older Android TV) is built into TVs from various brands, including Sony, Philips and TCL. It's broadly good and always getting better, but recent Google TVs have lacked UK catch-up apps and there's often a slightly disjointed overall user experience, with the Google TV OS feeling as if it's been slapped rather unceremoniously on top of the TV's own menu system.
Amazon's Fire OS platform, which many will have experienced via a Fire Stick streamer, is also now on various TVs, mostly at the cheaper end but soon also Panasonic's flagship OLED model. It pushes Amazon's own content a bit, but every third-party app you could want is there and the whole platform is very user-friendly.