Generally, it is whatever works best for your overall play style.
I tend to use most of the skills because each has its uses. I focus heavily on archery, then warrior, thief, and mage. I focus on archery to pick off enemies from a distance and this goes hand in hand with sneaking; you do extra damage this way. This means my enemies do not know where the attack is coming from. I generally continue to use archery until I am spotted. If two or more enemies approach me I sometimes cast area of affect damage spells, or I continue to use archery and then switch to melee when they get close. This applies to both outdoors and indoor environments.In small indoor environments I tend to do a little bit of archery, but mostly melee. Again, sneaking around as much as possible since sneak attacks causes more damage.
I don't use magic very often in combat unless I want to specifically increase my skills. I suppose that is mainly due to the fact that casting spells uses mana and I don't really want to bother drinking potions to restore my mana. Again, unless I am specifically trying to increase my magic skills. Since the game allows you to basically carry unlimited ammo I prefer using archery. Arrows and bolts have no weight and I suppose it is considered a cheat in away... especially when I am carrying 25,000 arrows.
Crafting and enchanting are powerful abilities in the game, but it is not absolutely necessary. However, if you want to create powerful weapons and armor then they are vital. "Crafting" includes both blacksmith and alchemy skills; alchemy allows you to create potions that temporarily boost your blacksmith / enchanting skills. Enchanting enhances the various weapons and armor you create (or non enchanted weapons / armors you find or buy.
With high archery skills (Marksman), a powerful enchanted bow, and powerful arrows as well as high sneak skill I can do one-shot kills on giants and mammoths and cause lots of damage to very high level monsters / NPCs when I ambush them.