I personally think the ranger is perfect for this "old-school" revamp AP. That being said, I think the slayer would make the most sense/best choice for the following reasons:
- brings a lot to the table in various abilities and customization
- makes for better roleplaying at the table in this AP (a ninja is far from home if they enter this adventure; that's for sure). A slayer can be roleplayed as a traditional, skilled adventurer, or you could go for the whole bounty hunter stchik.
- allows you to be a successful skill-monkey without sacrificing your combat ability
- dwarves are awesome.
- can be built as stealthy as you want, so you can still act like a ninja.
When you have a campaign that has a single race as its major enemy it is hard to beat a ranger. A dwarf ranger with favored enemy giants will work extremely well. Take the trapper archetype to get trapfinding and you can do anything a rogue or slayer could do. Favored enemy is a lot better than sneak attack especially when you max out your first favored enemy and it is the campaigns major enemy.