What happened to the arrow keys?? Why do people always use WASD. I hate those keys. But, left asside, I'll say what are the worst keys: ALT, CTRL or SHIFT. Gosh, I played games and that WIndows message keeps comming up because of reminant keys or however Microsoft calls them. Objectively, here's my thought.
I am right handed and cannot play well with WASD so hard coded "W" key to jump would be horrible for me because I just cannot play with WASD properly. I'm used to using a controller. So for me, Space bar is better. Now again, if you are using WASD for moving, the "W" key will also add additional checks in your code that a single jump only key would not. If you plan on hard coding keys for control, I suggest the "space bar".
But, in the way others said here, I most favor a user customized key layout. THAT would be best. You put your own personal keys as default but permit the player to change them. Yes, it requires saving into a file and reading from it, but creating a good games requires understanding your programming language INCLUDING saving and loading from files which some people may not be too familiar with. But what I loved about GML is that complicated things become simple when readin just their GML language manual and other users' examples.
So, your question here already has it's own answer with the current comments, we are all different and play differently which is the reason customized controls is best when using a keyboard played game. In the end, your game will know success with it's ads, it's visual appearance and story, level layout and sounds. If players really love your game, most will accomodate your hard coded keys but you may have some players send negative notes because they don't like the hard coded keys.
I think this covers most of the things arround what keys to use in a game. Of course, that's only my own opinion. Hope this helps!