How do I get the best quality?
Of course, there is no "best". There are tradeoffs, and better and worse choices, but there is no perfect set of encoding settings. End disclaimer.
The first step to achieving high quality output videos is to have high quality input videos. If possible, send us good quality video at the same size, or larger, than the output you want, and at a higher quality than you think you'll need. (Of course, if you're dealing with UGC, this isn't always up to you.)
Second, don't skimp on file size. Bitrate is far more important to video quality than anything else. If you ask for 1080p video at 300kbps, it's probably going to look bad no matter what other options you choose. If quality is a priority, setting video quality to 4 is usually a good bet. Audio quality 3 sounds pretty good, but 4 is even better.
Third, if you're using H.264, try using the Main or High profile. These are a bit more CPU-intensive, but result in much better quality video. But they won't work on an iPhone.
Fourth, there are a few advanced settings you can try. If your input is highly compressed and blocky, you can use our Deblock filter. If you want to squeeze in 5%-10% more quality at the same bitrate, try setting Denoise to "weak" or "medium".
Fifth, slow down the encoding. Set Speed to 1 or 2, which will allow our encoder to apply a few psychovisual optimizations that we don't apply on faster encoding.
Just remember that these are tradeoffs. Our default encoding, with no additional settings, looks pretty good. At the end of the day, you're making tradeoffs between visual quality, encoding speed, and file size.