Hi there – Nordica has this informative article: https://www.nordica.com/stories/the-seven-most-common-mistakes-skiers-make-when-buying-new-boots/?backstage=1
In general – a boot must feel very tight but without any hurting points (those will only increase while the overall tightness will feel less over time). The tighter/stiffer the boot, the more control and direct feel you will have. So eventhough we would always like to have comfortable, soft boots this will basically do bad for your skiing. Ski boots are just not comfortable – however you CAN find a boot that doesn’t “hurt” you.
Beware that there is a hell of a difference between cold boots and warm boots. Keep them warm until you put them on – never leave them in roof box in minus degrees and expect to smoothly slip into them… not possible, should not be possible if you have the right boot!
Many modern boots will be customized when you buy them – go for this and spend a little extra than just buying a standard-fits-all boot. You will regret it and it will still cost you a lot of money…