Which skills are best for a hero?

Leadership is key to victory for a balanced Necromancer, provided you can recruit him, and it's even easier if you have a mentor. If you read the manual describing the probability of recruitment, you'll understand what's what. This is especially true for recruiting native creatures (which hero and with which army you need to approach whom); Diplomacy plays an important role here, and if you have a Crown of Leadership, it's even better. Also, don't forget about the possibility of accessing inhabited mines, especially on large maps with many mines. In mid-game and rush strategies, Leadership is not worth taking; that's obvious. Regarding Lucretia in the late game, I agree with your choice, but only against powerful races with a mentor, as it's better to get rid of Sorcery and take Attack and Defense instead. However, against magical races, Zoltan is the best choice. Deydra is also good. Caspar is only good for rush strategies. Orson is good in the mid-game. There are many builds, and they depend on many factors (map size, opponent's race, game conditions, the presence of a mentor, the number of castles). So, the choice is not straightforward, and I'm sure there is no universal hero or build.