Let's start a holy war:
Tab vs Enter vs both for autocomplete / suggestions.
Please justify your choice.

Both but mostly enter. And i don't even know why tbh. I once had to work with an IDE not using the suggested entry when pressing enter but adding a line and it fucked me over so many times.

ctrl-n

Why enter? Enter is to execute the command i am typing or to add a new line. That is why is use tab or ctrl-n, depending on the context.

spaces ... wait ... what was the question?

Tab. Because enter is reserved for line break.

ALT+Enter is fine as it never gets in your way.

@Oktokolo agreed, the Holy alt enter

alt-f4

Well accepting a suggestion in most shells is tab by default. But I can have my mind changed.

Tab. The number of times I've autocompleted by accident when I just wanted a line break is pretty high.

@EmberQuill Same.
That's my answer, but I may try some of what other comments say...

Esc

tab all the way.

I still remember when I had to use Eclipse for a project and tabs didn't work, instead it sent the caret to nirvana. It just wouldn't let me continue typing so I closed and reopend eclipse instead of trying to figure out where it went. annoying af