When comparing this you need to determine the moves it learns and what you want to use it for. As lets go does not have abilities we will be using that as a con.
Dragonite
VERY BULKY and has a good attack. It suffers from low sp def which a lot of faries and ice types use. It is not slow by any means but is vastly out speed by most fariy types either being faster or able to take its hit. With out multicale it can't be a starting tank like it does with main series. It gets access to substitute with roost can make a nice set up to have a cushion.
Mega Zard X
With out tough claws, its moves are not as powerful as they should be. It does however have the chance to be both useful for faries and other types. As fire is resistant to fairy and once you knock out the fairy you can mega to either confirm a kill or mega after to be ready with more resistance. However, Zard suffers from more special attack and needs more TMS to help with that which early game isn't too easy.
Maga Zard Y
With out drought it will suffer but will up your sp atk coverage. Without stab sun fire attacks or solar beam, it can be very harsh to have a bad matchup. This also has the 4x weakness still to rock. This however still doesn't mean its high special attack moves arent gonna hurt.
In the end, X and Y suffer from the fact lets go changed the battle mechanics. For a casual play though it depends on what your team needs. Dragonite is a more safe/normal option to take. For competitive, they all have their own reasons to be on a team. You just need to know yours.