is there a best level to do guild quests so you get better gear or does it not matter

Some items relating to guild quests are leveled, meaning that you'll get the strongest version of that item only when you character begins that quest beyond a certain level.

For instance, one nice item that you'll find in a Thieves Guild quest is its strongest version when you start the relevant quest (or enter the relevant location) when you are level 46 or above. If you enter the relevant location at a lower level, you'll get a weaker version of the item.

But in general, it varies between guilds and quests and items.

chillrend is a leveled weapon which is most powerful weapon at 46 level its base damage at level 46 is higher than daedric sword plus has a cool enchantment i love that sword also nightingale armor and weapons are leveled items which is most powerful at level 46

I think the only Guild with leveled equipment is the Thieves Guild. The Nightingale Armor varies on level, and to get max everything (Armor and Weapons), you'd need to be level 46+.

I'm not sure about the Thieves Guild Armor itself. And Chillrend, which is acquired via Thieves Guild, and level for max enchantment is level 46+.

Ok thanks, I was thinking of doing the theives guild because they have a nice armor set, I will wait until im past level 36 to do so. I just started a new toon and I am level 20.

thanks everyone

As said, some of the best items are leveled. Meaning at 46+ you'll be getting the best versions. The Thieves Guild has the majority of these leveled items.

Thieves Guild and Nightingale equipment cannot be Smithed with a perk to increase their strength... which is a shame.
Therefore the Nightingale Bow and Sword immediately decorate a weapon plaque on the wall.

I do like my Nightingale cuirass however (even though it's weak and doesn't protect very well).
Any common light armor (ie. Scaled) can be Smithed way stronger.

The nice blue/green sword is a different matter, though!
As long as you are Level 36 + you'll get a good version of that weapon...
There are reasons why it's quite famous - it's a wonderful sword.