Necro is hands down the best solo spec in the entire game due to its crazy sustain via shroud, minions, etc. while having more than reasonable damage output and access to easy might generation. It's also versatile in its weapon choices, you can either go melee or ranged, or a hybrid of those two. Overall it's really well-rounded spec. Warrior's propably the second best solo spec. Just like necro, it has very high sustain, but I'd say its damage can be significantly higher. Warrior has some amazing, stacking, easy-to-access damage modifiers, while being able to give himself stuff like easy 100% crit chance and quickness. But it's also a full-melee playstyle, which comes with its own disadvantages, whereas necro has none really.