In general, you will want your conversion page to be a page where users complete a defined action that indicates business results for you.
If your site has a lot of products and you're trying to test overall conversion improvements, you might want to use a 'Thank You' page as your conversion page - this will enable you to capture any successful action users take. However, if you're trying to test completion of a unique goal, you can narrow your focus with a conversion page that's unique to a specific product - for example, the purchase page for that product.
The more actions it takes a user to get from your test page to your conversion page, the more traffic you will need.
