Which is the best legend and why?

Heavily depends on your preferred playstyle.

For a face to face encounter, Maggie/Ballistic is really good. Maggie's Q counters doors/walls too, so make use of it. For Ballistic's ult fast firing weapons get an especially massive buff with faster reloads combined with infinite ammo, but any gun can work with it.

If you prioritize your ring position, Path/Valk could be really beneficial. If you enjoy snipers more, try Vantage.

Conduit lets you give temporary shield heals, so useful on a lot of situations.

My personal favourite is Mirage, but it's kinda tough to get the hang of him. If you can play in a way unexpected by the enemy team he's really good, but if you get read you're fucked.

For defensive play, Wattson is quite well. By far the best character for defending an area, including an actual shield generator thay refills using a single accelerator. Caustic could be used too if you like to play aggresive at times.

If you enjoy machine guns more, try Rampart. Her walls give you open cover to fire your machine guns which is pretty good.

I think whichever one is at the apex would be the best.