Depends on what you are planning for that. If you plan to use it at work like DJ-ing or plan to put in a show car. 15inch or bigger. If for personal use. The smaller the better.
The subwoofer with the best sound quality is the best, and size depends on your needs. Don't buy sth too big for your home without SSS-tier soundproofing, and your neighborhood may want to call the police.
Sub-woofer: A small dog. Anyway, there are a lot of things you need to tell us before we can recommend anything. For one, are you living in an apartment or a standalone home? Basement or above ground? Soundproof walls/sound dampening walls or echo chamber? Neighbour with shotgun or open space? Oh and what are you going to use it for?