I've got a question.

Let's say I want to upgrade my Nightingale Bow. I know that I will need the Arcane Smither perk but do I need any other perks to make the bow flawless?
Or do I just need to level up my smithing?

100 smithing will give you flawless with no perks taken. But, there are some daedra lord artifacts that cannot be improved beyond flawless even if you have taken all perks.

With the perk and high lvl smithing it'll probably go up to Epic or Legendary. I'm not sure what the base type of bow the Nightengale bow is but I'm sure it's easy to find out.

Practically (or virtually) all items can be improved to Legendary, provided that you've 100 Smithing and you get powerful Fortify Smithing stuff (including Blacksmith potions (the lesser)/dreughs/elixirs (the best, at least so far I've found), although multiple potion effects aren't cumulative, meaning that only one potion will have effect at a time). Even more, if you find even more powerful Fortify Smithing stuff in the future, you can upgrade again your stuff even if it's already in Legendary level.

Although there are no perks for Leather or Iron, I've my Leather Armor improved to Legendary (+ another upgrade that I did recently), for a total of 474 AR (with all Light Armor perks) out of the theoric cap of 567. And I've also upgraded my beloved enchanted (iron) pickaxe to "Legendary+", too

You're only limited to "Flawless" quality (which is the middle grade quality) if you do not take the perk to lower the skill requirement for the improvement type.

Without the perk (can't remember the name), Epic requires 150 Smithing and Legendary requires 196.

With the perk, Epic is only 80 and Legendary is 100.

With the perk and 100 smithing, Daedric can be made legendary without any fortification of skills.

Something is wrong with that. There are no perks for Leather, and it still can be http://steamcommunity.com/id/Dovakhan/screenshot/614969358074370639?tab=public, and beyond. Same for my enchanted iron pickaxe.

I thought Epic was the best?
My Dragonscale is legendary, while my Nightingale armour is Epic.
I maxed out smithing and took all perks.

What did I miss?

Yeah, I'm curious about this too. All the Daedric stuff I made and improved is "Legendary." But some things I upgrade are "Epic", some "Flawless", etc.

Well I'm not sure of the exact numbers but I am currently running around in Legendary Ebony Heavy Armor. And my Smithing is at 85.

After I got 80, I spent a perk on the ability to make Ebony Armor. I could now upgrade my Ebony gear to Epic. Before spending the perk, I could only upgrade Ebony to Flawless.

If you want Legendary, pop a Blacksmithing potion and quickly upgrade all your Ebony gear in 30seconds to Legendary.

Note: You can go straight from no upgrade to Legendary as long as you have the mats, spent the perk and popped a BS potion

You both miss using Fortify Smithing stuff.

In order to improve my Leather armor and my (relatively) pitiful iron pickaxe to Legendary+, apart from 100 Smithing I also used a Blacksmith Elixir (+50% quality to Smithing), gauntlets of +20%, a ring of +20% and an amulet of +25%, all together.

I have 69 smithing skill +15% smithing ring and +20% smithing necklace and can upgrade everything to legendary.