I am running pi hole, and was wondering where the best placement for it would be on the server; the lan interface or the wan? Is one safer than the other?

EDIT: Also, I have dns servers defined in my interfaces file. If I want it to hit the pi hole first, then somewhere else, should I not have dns servers defined in the interfaces file then? will this bypass the local pi hole dns server?

You know your network, but we don't. Such things are best done with a full chart of the network to hand.

You want the DNS server serving all the network I presume, and not the world. At the same time it may refer outside and should receive answers, and updates. That pins it down to one or two places. You might be able to dispense with it altogether and use external services as the internet is well supplied with free services.

It would only be serving internal requests. I assume the best place for this is the lan interface. My setup: internet => server (wan interface) => same server (lan interface) => WAP => wireless clients.

If you read the docs for Pi Hole, you will see that it needs to be "THE" DNS server on your network. Period.

As far as your client devices are concerned, the ONLY IP address(es) they should get as DNS servers are those associated with your Pi Hole devices only. The Pi Hole allows you to configure "upstream" DNS servers, and in here you point to your LAN-based DNS servers that a) can resolve your local network information and b) either forward out to another server (your provider, Google, etc.) or are TLD-aware.

ANY OTHER SETUP leaves your client devices potentially exposed as they could request information from servers that do not contain the Pi Hole logic to block the advertising domains.

And, you want the device inside your edge device(s). Inside the router and any firewall devices.