From my purely anecdotal experience (selling 10 or so iPhones of my own and my wives since the original) they seem to loose the most in the first year.
Speaking just about the lowest capacity $650 (unsubsidized price) phone, they tend to sell for $350 (give or take $20 for condition) after a year, or a $300 loss from new. After year two you could maybe squeak out $200 or so; a $150 loss in year two.
All this assumes craigslist private sales (not using gazelle or the like who will resell) and a phone in good to great condition, has its boxes/cords/etc. I am also ignoring carrier subsidies as they obfuscate the true value. In fact the main reason I upgrade each year is because I can do so at almost no additional cost ($100 or less) when accounting for the subsidy.
So the answer is very much "it depends" when talking resale. YMMV