This is a big question because there are many different soccer leagues, each of which may have its own card manufacturer. That said, the two largest soccer card manufacturers right now are Panini and Topps.
Panini makes the official cards for the English Premier League, which is the sport’s most popular domestic league. However, Topps makes the cards for the Champions League, which is the premier European soccer competition. Topps also makes the cards for the Bundesliga, which is Germany’s soccer league. Generally speaking, Premier League and Champions League cards have the largest audiences. Champions League cards also offer access to players from all over Europe, rather than restricting you to a single country’s league.