Is top floor balcony good with no roof?

Check and see if they would let you install a retractable canopy. I have seen it done before and this could afford you the best of both worlds. It's certainly nice to be able to enjoy direct sun when the weather permits.

Is the 6th floor is the top floor of the building?

Those retractable awnings are great but you have to be careful with them; if the wind catches it & bends it, it may be no longer retractable. Caveat emptor.

Thanks for the idea about the retractable awning.
Technically the 6th floor is not the top of the building, but there are no balconies above it because there is a large terrace right above.

at least you can grow veggies with full sun, for part of the day at least.

Unless you fully enclose it, anything you put out there will be subject to the elements, with or without another balcony above. Rain doesn't use a plumb bob. Neither does soot.

Just what do you plan on putting out there? I recommend heirloom tomatoes (especially Cherokee Purple), but you need the right light. Or, maybe, stuff that's made for outdoor use.

Hopefully no one will toss cig butts from the terrace onto your balcony...

The key point is the ability of someone above (either via balcony, roof, etc) has clear access to your balcony. As stated, the tossing of cigs, bottle caps, etc can not be overlooked. This is a huge problem in my building where there are numerous exposed terraces on multiple levels. You would think that people of certain status in society would be more considerate, but it actual appears to be the opposite.

So, if exposed, go with the covered option, otherwise sunlight and open air wins.