Logistics - makes a huge difference, probably the most useful skill
Armorer - great for everyone, can become pretty op with heroes that have it as a special
Offense - good for everyone, another must take
Wisdom - if you want to cast spells higher then level2, protip: you do
Nacromancy - op, you will start with it
Earth magic - this is the magic skill that you should always take, irregardles of who you are, best magic skill by far
Leadership - I don't like skills that are based on pure luck, but it's not half bad, can win you a battle if it triggers, many "good" heroes start with it, is a wise choice if you plan on taking mixed armies
Diplomacy - some think it's op, personally I don't like it, but you can easily bumrush your opponents on smaller maps if you have it, good combo with leadership
Luck - same as leadership, but there are no penalties for negative luck so it's less worth taking
Artillery - good for creeping and defending a castle against weak wandering heroes, useless in bigger battles
Ballistics - you will miss anyway
First Aid - probably the most pointless skill in the game, avoid
Eagle Eye - battles First Aid for the "holy shit do I suck" title
Pathfinding - can be a godsend on maps with lots of shitty terrain, especially for naturally slow armies like necro, very situational skill
Navigation - the bonuses it gives are freaking huge, but I still fell it's kinda of a wasted skill slot
Archer - good skill for Tower, shit for Inferno and Fortress, so so for the others
Estates - the only time it's useful is when you play on impossible and get a secondary hero with it early on, otherwise the amount it gives is way too low, funny because this skill was a real godsend in II
Scouting - pretty pointless, even for secondary heroes
Learning - um, if you want to bumrush someone then it has some potential to help, on second thought - no, it's shit
Intelligence - good skill if you're basing your build on casting lots of high level spells like ressurection or implosion, avoid otherwise
Misticism - this would be a decent skill if you doubled or trippled the amount, rather pointless in its actual state
Sorcery - for dedicated nukers
Water magic - gives mass cure/dispell, also has best buffs
Fire magic - best debuffs, many of its best spells are pretty difficult to get, contrary to what you might think it has absolutely shit offense bar the infamous armageddon spell
Air magic - least useful school, but still not the worst skill you can take all things considered
Scholar - good on a support hero on huge maps
Resistance - can become decent when supported by certain artifacts, still a bit meh
Tactics - has its uses, but holy shit is it irritating or what