What's a good substitute for basil in pesto sauce?
If we're talking about the big classic pesto alla genovese, then unfortunately...
There is no substitute.
Basil is the majority ingredient in pesto.  None of the other suggestions here will taste even remotely similar.  You'll be making a completely different dish entirely.  It will be some type of vegetable/oil paste, but it will not taste anything at all like pesto.
It would be like trying to make baked beans without beans, or french onion soup without onions.  These ingredients are so dominant and so essential to the flavour of the dish that there's literally nothing left without them.  Just forget about it and make something else.  It won't be pesto without basil - not even close.
Critically, if you're using this pesto as a component in a larger dish then
it definitely will not come out correctly with any of these substitutions.  Pesto is used in a lot of ways and every recipe that asks for it is expecting the flavour profile of a basil pesto.  No substitution will be effective in this case, and each recipe that calls for pesto will have its own unique flavour profiles that will clash in various different ways with each of these substitutions.
If you're desperate, whatever substitutions you do end up making will depend heavily on what you're ultimately doing with the pesto - what you're pairing it with.  We don't know what that is, so it makes it that much harder again to make a reasonable suggestion.