For the Knight, there's no point in leveling up the ultimate ability; it's inherently weak, and there's nothing worthwhile along the way.
The Ranger's ultimate ability is good (though the Dwarf surpasses him), plus there are useful skills along the way (starting with Logistics and ending with Storm Wind).
The Mage's ultimate ability is very strong, but it's difficult to reach, although it's beneficial (Spell Lore and Enlightenment).
I most often took the ultimate ability for the Dwarves; it's incredibly powerful! And in addition to the ultimate ability, they gain amazing skills (Destruction, Summoning, and Readiness).
The Necromancer's ultimate ability is also powerful and fits well with their development (Enlightenment, Summoning, and Darkness).
I leveled up the Warlock, but it didn't impress me much, although the skills leading to it are good (Sorcerer's Luck, Spell Lore, and Destruction).
I haven't tried the Demon's ultimate ability, but it seems that for it to be effective, you need to take all the skills in Summoning, as well as those in Luck and Leadership. The required skills are also not bad (Logistics, Darkness, and Attack).
The Barbarian's ultimate ability is one of the strongest, as the Fury bonus table contains many good things, and the skills for the Barbarian are decent (Leadership and Attack).