Gonna replay this game before going to HoMM3 again.

What is the best combination of skills a necromancer should have to be very powerful at combat?

I remember I loved the revive (or something like that) spell, plus the teleportation spell could make a hero very powerful. Since it´s been a long time I first played this game I don´t remeber very well how to proceed.

Been a while since I've played too, but I'd probably pick the from the following:

No-brainers:
Earth Magic - best skill in the game. Mass slow, Implosion, Town Gate (the I Win button)
Necromancy - hordes of skeletons!
Wisdom - for casting Town Portal, and whatever other spells you happen to get

Useful:
Intelligence: more spell points is more spells cast
Tactics: good for melee heavy necro troops
Pathfinding/Logistics: move movement is always useful
Air: Mass Haste, Air Shield, Dimension Door, other useful spells
Luck: who doesn't want more damage?

The OP asked about HOMM2, so the magic schools and new skills don't apply here.

Comprehension would probably be an excellent one to start with then.

Ahem

In which case, having Wisdom and Necromancy as starter skills, actually not a lot different (barring magic skills)

Good:
Pathfinding/Logistics - movement still good
Luck - damage still good

Moderately useful:
Ballistics - great early/mid for scooping up castles more easily with not many fliers/ranged
Archery - not really required with only 1 ranged unit, but why not
Navigation - if on a map with lots of islands
Estates: money is always useful

Maybe:
Mysticism: if there you're playing raider and there's not many wells around
Leadership - only if you play on taking any other racial units along in your army. Otherwise forget it

The thing is I am playing the campaign mode and am trying to avoid skills that prove not that useful in the long run, like estates for example, as it is ony useful if you have very little resources on a map, nad occupies a skill slot which could be better used for something more useful in the long run.

The thing is, that in Heroes 2 is much less skills to choose from.
The base skills you already have (Necromancy, Wisdom). Other you can choose on what Creslyn pointed out.
Logistics is always a good choice, Pathfinding and Navigation are map-dependent.
Take Archery as a Necromancer only if you insist taking another ranged units from other towns (in that case Leadership may come in handy).
The less towns on the map (and the more mana-hungry spells like Dimension Door you use), the more useful Mysticism will be.
Ballistics, Luck, Estates, all are useful

I thought that Necromancers couldn't get Leadership without visiting the crazy witch lady in the hut?

Whatever the case -

Estates is best if you pick it up early, and let it do its work over the course of the game. It is, actually, a lovely skill - one month of expert Estates will give you an additional 14,000 gold to play with, which is not insignificant at all. And that's per hero. I take estates for the same reason that I build statues quickly - in order to keep my finances as far to the red as I can.
The only flat-out "ignore" would be Eagle Eye. I don't understand its purpose to begin with, and its chance of working is too small for it to be a genuinely useful addition to your skillset.
Scouting is a skill of heavily diminishing returns; take it early, if at all, and you really only need it with one hero. (the addition of a fog of war in Heroes IV changes that, but we're not there yet! Even the necromancer's Cover of Darkness in it third game gives Scouting more long-term viability)
Mysticism is most useful if you're throwing around small spells or you have significant breaks between fights. An additional three points of mana might make the difference for that last-gasp lightning bolt or haste. Maybe. But it's unlikely. When the fight is coming hard and fast, you probably want that slot for other things.

Luck is a must-have (for me anyway) if at all possible, especially since you can't get double-move Morale bonuses with the Undead. Archery is always nice, but not necessary for the shooter-poor Necropolis, and mixing units is harder with them than with any other. After that, it's kind of a matter of preference. I never take a Wisdom upgrade unless I have the resources (i.e. mage tower) to make it worthwhile, unless the skill offered opposite is one I absolutely do not want. That being said, Expert wisdom gives you access to fifth-level spells, which DO matter. Town Portal makes multiple-town defence infinitely simpler, for example.

I began to play again the Necromancer campaign.

I realized the skills of the heroes don´t pass on to the next scenario apparently. So the long term skills don´t make all that dfference.