This is purely my opinion.
All of them.
For me, 4e was the breaking point of conversions. It's not that its impossible, but the game works under different mechanical assumptions than previous edition in terms of spellcasters scaling, etc...
The important thing to remember though, is that the AP's aren't necessarily just an adventure in and of themselves. They are also guidebooks to the setting. For example, while Second Darkness gets SLAMMED a lot, the first adventure does a great job with Riddleport and a further adventure does a great job with the dark elf city.
Look over the themes of the ones that sound interesting and move from there.