Started a Legendary playthrough, and I figure to increase my chances of not failing miserably I need to utilize trainers.

So basically, what skills do you find to be most difficult to raise?

I am thinking of training in smithing since raising it has become a pain ever since they patched the hide bracers thing.

These are the ones I find the most painful to level up:

Lockpicking

Pickpocket

Heavy Armor

Light Armor

Block

The bottom three are just a pain to level up by getting hit over and over again. Lockpicking doesn't give you experience for picking a lock you've already picked, so at some point you'll have to just break picks again and again and again just like you'll have to get hit by enemies again and again and again to level up armor and block. Pickpocket is difficult for me just because I'm not sure what makes the NPCs respawn the items in their inventory.

There's an argument hat says you don't need skills you're not using, so why bother training them? However I find that some do need it from time to time. For example, a successful archer won't often allow anyone to get close enough for melee combat, so armor skills will be slow to raise. In those cases, training to catch up makes sense. I still wouldn't suggest training until you find you need it.

Smithing is likely to be one that might not get used much, if you find gear as loot that's better than you can make. But that falls off as you progress, as all the loot is untempered, and the enchantments don't make up for the base damage. Smithing is therefore not so useful early on, but you'll want it later.

One approach is to just get the Arcane Smithing perk to let you temper enchanted weapons, and use potions (and items enchanted with Fortify Smithing) to make the other perks unnecessary. You can get the base damage of a steel weapon up close to a Daedric one without investing in all the other perks, as the tempering improvements are added points, rather than percentages,so the difference in base damage rapidly becomes insignificant. I have not done the calculations, but levels in Alchemy and Enchanting may give a better payback in weapon damage than levels in Smithing! Andif you're making other use of those skills, it's a no-brainer.

Train the skills that you will be using.

And Smithing, so that you can craft Daedric/Dragonbone!

Nothing says "Look at me, I'm the Dragonborn" like wearing the bones of a Dragon you've just slain!

The reason I need to train is because I want to get to a higher level to where I can actually see some of the higher dragons and enemies. My main character is pure stealth and has maxed every skill he uses, and so he is stunted somewhere in the 50s. Plus, more levels means more perks, which means a stronger character.

It also makes some sense to train primary weapon/ armor skill as it makes you more lethal.

On lower levels its also an cost issue.