I do a lot of AI upscaling, specifically 1080p to 2160p with Topaz VEAI, and its quite good. That said, it isn’t magic. It requires a really good source before hand (avoid anything with film grain or has been compressed) and you do not want to make a giant leap. The key idea is the more you tell the AI to fill in the gaps, the more you will get AI shenanigans. If you have a good clean BR rip, then the AI can bring it up to 4k quite easily.
what is the definitive best way to upscale to 1080p in 2023?
1080p to 4k is quite easy, but getting something to 1080p is massive mountain to overcome. Like I said above, you want to leave as little to the AI as possible. Bringing something sub-HD to HD means entrusting the AI with a lot since there isn’t much detail to begin with. My best efforts have been 720p-1080p upscales, but I have had some good results with upscaling the 1990s Sabrina live action series and off the top of my head that is 480p. If you look really hard on those upscales, you can see some strangeness like how words in the back ground are just gibberish or maybe a hand doesn’t move exactly right, but it is a vast upgrade over the terrible DvD release with its low resolution and interlacing.
If you are going to upscale, I recommend Topaz VEAI. Its really easy to learn (I went from having no video editing experience to being able to do some competent upscales in three months), its a lifetime license with a year of upgrades, and frankly its the best on the market. The only drawback is like any AI, you need a hefty rig. I am rocking a 3080ti, i9 processor, and 128gb of RAM. Its also very time consuming. My hefty rig can do the upscaling part in about 45 minutes, but even taking the mild tweaking in DaVinci out of the equation, encoding that same episode is going to take a good 11 hours unless you want a 200gb episode on your Hard Drive. (Please do not trust Topaz to encode for you, its a good upscaler but its better to let Handbrake do your encodes). Throw in the deinterlacing I do before I even run it through Topaz, and it can take me 24 hours of work spread out over a week to get an episode done.
If you have any specific questions, I will be happy to answer them.