I went to the shops and found an iPhone 6 on the ground?

The screen is really cracked and stuff, should I keep it or give it back to front desk?

Keep it why? Turn-it in…

Give it back, its not yours.

Okay so where is it now?
At your home? Think about it, if you lost your favourite thing, would you want it to be returned or taken. That guy who dropped his phone is probably crying sadly at home. I know that feeling, I dropped my Phone last week, I saved my lunch money for 6 weeks to get that, I was feeling so bad until someone ring me up and returned the phone to me.
Even it is cracked, you should return it, it is about honesty, you will be surely joyful for the good deed you did

I'm not gonna be the guy who says it's not yours and what not, but here's some information about Apple's security:

Apple uses iCloud which a user can remotely lock the device and also track the device using GPS, if you decide to keep it then you're leaving yourself vulnerable to either having an angry person at your door demanding the phone back (maybe with physical violence as well) or you will have the police at your door with a warrant to search the premise for stolen property. Or they could simply iCloud Lock it and then it will be completely useless, the lock can't be bypassed so there's no point in keeping it

Return it since it has probably been disabled and you won't be able to use it

That might be someone's phone who did a crime and is being traced so he threw the phone which caused its screen to break. From my opinion leave it there as it was or to the front desk

You SHOULD know the answer to this question yourself. See If you can make out a phone number… Usually the first one is most important. Say you found it and its here if you wish to pick it up.

Doesn't matter if you keep it because iphones have icloud lock.

You should definitely give it back. It belongs to somebody and someone is going crazy over not having it. Give it to the desk so they can try contacting the owner, and you'll be rewarded for your good deed in the long run.