Since the class system in Elder Scrolls Online is pretty different from your average MMO, determining the best solo class is almost impossible. Each of the classes in ESO can fill all roles whether it be tanking, dpsing or healing. They all bring something unique to the table as well.
In Elder Scrolls Online it does not come down to what class is the best at soloing, it comes down to how good you are at soloing. How you build your character is also important. Someone who groups or raids a lot will primarily want to build for one of the archetypes (tank, healer, DPS). With someone who wants to solo, you will want survivability and damage output. If you want to level quickly, the DPS is more important. If you want to solo hard mobs, survivability is more important.