The most annoying and troublesome thing in Morrowind (and Oblivion) is how the levelling system works. Or arguably doesn't. You actually do not want skills you'll be using all the time as major/minor[footnote]with very few exceptions where the skills is super hard to level up otherwise, like Mercantile[/footnote] otherwise you cannot control your levelling efficiently and you'll level too fast. Which may sound great but it's not because of how the attribute bonuses are calculated - if you are levelling really fast, you'll be getting quite crappy attribute improvements. Weapon and armour skills are particularly bad at this because you have completely no control over them and you may just gain 2-3 levels when you go somewhere to kill some stuff, which means even worse attribute bonuses.
I heavily recommend getting a levelling overhaul mod, like Galsiah's Character Development (GCD) [http://mw.modhistory.com/download--6955] or MADD Leveler [http://mw.modhistory.com/download-68-11157]. They broadly do the same thing but in different approaches. The way I'd describe them is "they make the levelling like Skyrim" because you stop caring about the skills and what to level and just play normally - you just get attributes as you level up skills naturally. By the way, I'm saying "like Skyrim" in quotes, because it's actually the other way around - Skyrim makes the levelling like them[footnote]Ah yes, there were many people who criticised Skyrim for simplifying the levelling system too much when...in reality some of the most popular mods for both Morrowind and Oblivion are ones that make levelling simpler and it was Skyrim which took after them.[/footnote]. I've used GCD myself but it doesn't really matter that much which one you go for.
Other than that, if we just take vanilla skills
- Long Blade - it's fine. It's probably one of the best wepon skills in the game. Not necessarily "OP", but long blades are the easiest to find.
- Block - this one is really good, actually. It's also one of the skills I'd place as major or at least minor if you're going to use it. Reason being that your block skill gives you a block chance - and that's the way you use it - it automatically triggers on some attacks. And the way you level up the skill is by blocking. So if you don't block (because your skill is low) you can't level it up as much.
- Heavy Armor - as long as you pick an armour skill and stick to it, you'll be fine. The only thing here is that you need a bit more strength to carry the armour, but you can also enchant the armour with stuff like permanent feather (you can carry more, so it negates the armour weight).
- Armorer - useful for combat types. It's a skill you can safely put as major/minor as you can also control how it levels.
- Restoration - it's a fine skill. Provides healing and stuff.
- Destruction - probably not that useful if you are not going to be slinging spells all the time.
- Alteration - other than unlock and shield, it doesn't offer that much. You can also get the effects through Enchanting, so...your choice whether to get it. It's not something I'd expect you to be spamming, though.
- Mercantile - yeah. It's not that necessary, as eventually you'll be swimming in money regardless. You can also do some tricks like selling arrows one by one (1 arrow = 1 septim, but if you sell 200 at once, you might get paid 150 septims). At any rate, Mercantile is a ***** to level up anyway, so whatever.
- Speechcraft - it has a similar problem as Block in that it's chance based, so getting a boost is really useful. It's a really good skill, too, even if there isn't really anything to convince people of - you can raise disposition (which in rare situations is what you need to get some information) and you can also taunt people to make them attack you. Protip, if somebody attacks you, it's not a crime to kill them and take their stuff.
- Alchemy - the skill breaks the game if exploited. If you don't exploit it it's still useful. For an somebody using Atronach it's probably a bit more useful, as you will need potions to refill your magicka.
- Enchanting - it's super useful for class. I know you were wondering whether you should replace Block with it, but I'd suggest dropping something else in favour of this skill. When you get enchanted items (e.g., sword that does fire damage on hit) then Enchanting determines how much charge this uses and also how fast it recharges. It also determines how much charge you get from using a soul stone on the item. It's really useful because pretty much everybody will be using enchanted items. It's also another skill that's quite hard to level up, so getting it as major skill is quite beneficial.
Other than skills, you also have the attributes you get at the start. Frankly, they don't really matter. You only get a slight bonus. Skills are way more important. With that said, there are two attributes that are "the best" to pick up - Endurance and Luck. The former because more HP and Endurance level ups are not retroactive, the latter because there is no skill that provides has Luck as a governing attribute, so increasing Luck is hard. It's not crucial, however, so don't feel like you have to max out Luck.
