I'm writing some shell scripts that are intended to be POSIX-compliant enough to work in BASH, DASH and ZSH.

What should I use for the shebang?

I assume it's the following, but I thought I'd ask here first:

#!/bin/sh is probably the best way to go, at least when bash is symlinked to /bin/sh and is called as that it will run in posix mode and disable any bash specific functions.

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What OSes are you targeting?
Are you sure at least one of the above mentioned shells will be available on them?

I'm targeting Linux. If there's a Linux distribution that doesn't provide one of the three, then I don't need to support it.

Make sure they run on ash/dash, and then you can use the '#!/bin/sh', if they don't run because they need bash features then use '#!/bin/bash'. I would use:
http://sourceforge.net/projects/checkbaskisms/
with '-p' and maybe '-x' to ensure POSIX compliance.

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From the shells I've tested, 'dash' was the most minimalistic (or most strict if you wish), (an example: it doesn't accept trap '' SIGHUP only trap '' HUP)

#!/bin/sh should do what you need, i think the newer release supports arrays like features too, at least it worked when i tested on my test machine.

@SAbhi You are missing the fact there is almost never a native /bin/sh under Linux. On all mainstream distributions, it is a link to a Bourne shell compatible implementation, commonly bash or dash, less frequently zsh or busybox.

Dash doesn't support array variables and probably never will (unless the POSIX standard decide to add this feature).

I am probably going to cop a flogging for this, but are there really that many distributions that do not contain a copy of bash?

I am aware that it may not necessarily be the system default, but I did not think this implied it was not available at all.

Looking at some main stream distributions like, Mint, Ubuntu, Debian, openSuse, Slackware, I was not able to find any that indicated bash was not included.
I am not saying that posix standards are not the way to go, however, I find the argument to steer away from bash on most modern distributions to not necessarily
be a good argument either.

Just my 2 cents of course

What I can say I have noted as a good practice of late, is that rather than setting the shebang directly is that you let env find it for you.
As an exmple:

As has been shown by recent shellshock bug, relying on a big piece of software when you don't have to may bite you in the ass. Also, there are some distributions that do not contain a copy of bash, for example all those based on busybox.

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On most of the systems, Yes it is bash and in many but less to former cases a link to DASH.
based on the fact DASH doesn't support arrays but bash does.
Sorry for not being illustrative in my previous post and thanks for correcting me.