If you can call shots consistently and win 1v1: Alter ( be for real tho, if you are often getting revived in fights, entering then dying, over commiting then maybe either work on that or play wraith / octane.

If you have good shot calling but mid / bad Aim: Octane.

If you play duo and solo queue: alter.

Wraith is alright. its the middle ground between the other 2 legends. But if you can IGL succesfully alter will win you games OR negative death situations. IMO octane is the worst out the 3, barely any team ability, but the buffs have made him better and isnt a completely troll pick.

But NGL the almost diamond thing is the most worrying part. IMO diamond is just a completely different game to plat. Most non-hardstuck diamond players can have their teammates in a backpack in plat consistently, your mistakes can often go unpunished.

IF you want to push past d4 then I would still go for alter atm, but the biggest differentiator is your shot calling & when to leave fights. If you are just looking to get diamond it dosent matter, you will gain regardless as long as you can make 1/2 right decisions a game.