Yeah Frank, its a great series of little films and I can't believe I didn't discover these films earlier.
I think all of the films have their strengths and weaknesses and I like them all about equal. But I think I like Night of the Seagulls the best because I'm a sucker for Lovecraft influences and I thought it came the closest to actually having a plot LOL.
I think Tombs is my second favorite, great atmosphere and creepiest. People say that there are alot of plotholes but I think they can call be explained.
I like Return because of its fast pace and great soundtrack, but didn't care for the large crowds, fistfights etc...
Ghost Galleon is considered the weakest but if you take away the "toy ship floating in a bathtub" shots I think it compares quite favorably to the other movies. Perhaps the plot is marred by a few gaps in logic though.
I think De Ossorio is a hugely underrated director. I would like to see how the likes of Spielberg and James Cameron would fare with a similar endowment of resources and budget.