The Physics engine one would probably be popular. Slopes and advanced platforming is covered to death elsewhere, and also there is Noel's Advanced Platform Engine that does a good job of showing how to do it.
I think the best tutorials are 'how to create x game' ones. They tend to start off covering basics, but as the parts go on they cover specific topics naturally as they come up. An RPG would be a good one; I don't know that anyone has done one of those for FlashPunk and it covers a wealth of topics (grid-locked movement, NPCs, menus/dialogues, particle systems for battle effects).