The best solo class in Diablo Immortal is different than just asking for the best class. In groups, support characters, dedicated tanks, pure healers, and mobility units all can play a role perfectly. but what class plays all these roles the best?
It's fair to make an argument for another class in Diablo Immortal besides the one recommended here. Veteran players on top of their game have very little consensus on this issue. If they are arguing about who the best is, then it's fair for personal preference to play a role. But, statistically speaking, there is a class rising to the top consistently.
The Case For The Necromancer
The Necromancer has already established itself as the best class for beginners. Summoning units take care of the tanking, allowing the Necromancer to focus only on dishing out death and moving to avoid damage. But the class is more than just a fast way to level up.
In the endgame, the legendary equipment and item sets allow for a level of customization to tailor the Necromancer for any challenge. Beefing up the summoned units, spells, melee damage, or disables are all on the table and can bring the Necromancer to an elite status in any individual category.
The Proof
Hopping around the various servers, it's clear that the Necromancer, with some competition for the Wizard, is leading most of the challenge rifts and battlegrounds. Yes, teammates can help with these, but it is important to note that the Necromancer is supreme in both solo and party when searched.
It should be noted that the class tier list is so tight that many other classes are very close to catching the Necromancer. With a patch or community discovery, this whole balance could change. But, for now, the Necromancer is on top and, even if the class has its skills nerfed, the core concept of the Necromancer will always be the best, in general, for solo play.
Diablo Immortal is available now for mobile and PC.