I rotated at all of them as a medical student, but only lankenau for IM, so take whatever I have to say with a grain of salt.
I'd rank them Lehigh valley > lankenau > st. Luke's
Lehigh valley is a pretty nice hospital. It's clean and most of the equipment is new. They have a strong focus on education, at least for medical students. I did 2 rotations there and they were both very organized. The residents I worked with were knowledgeable. They have 5 internal med fellowships are I believe they often take their own. The area is pretty boring, though. It's a relatively small city. Not much going on. It is close to st. Luke's, so the surrounding area is basically the same for Lehigh valley and st. Luke's.
Lankenau is decent. It's smaller than Lehigh valley and the cases are probably less complicated than what you'd see at Lehigh valley. All of the complicated stuff gets set to penn, Jeff, temple, etc. They have morning lectures and noon lectures. The residents seemed knowledgeable. They have lots of fellowships, but I don't think they give preference to their own residents. They seemed to take people from the surrounding university hospitals as fellows. The hospital is in a safe suburban area on the outskirts of Philadelphia. It probably would be more interesting than Lehigh valley or Bethlehem, PA.
St. Luke's is also fine. Even though I ranked it last of the 3 I don't think it's bad. St. Luke's is smaller than Lehigh valley and probably similar to lankenau. Lehigh valley probably steals the more complicated cases from st. Luke's. St. Luke's is also an older hospital and not nearly as nice as the other 2, but it's not a **** hole either. The case volume is kind of low st. Luke's, at least when I was there. I didn't think the teaching was all that great at st. Luke's either, but I was doing surgery there; medicine might be different. They have the fewest fellowships of the 3. The only "desirable" fellowship they have is cardiology and they haven't taken a DO for the fellowship for awhile (at least not in the last 3 years). The surrounding area is probably the worse of the 3. It's basically in the same area of Lehigh valley, it's just the poor part of town.
You should check out gesinger. It's probably better than all of them and they seem to give preference to their residents for fellowship. Danville, PA is super, super, super boring p, though.
Albert Einstein is also a decent aoa program.