I think we need to clearly define "foreign" player. Born outside the US or no US college.
Foreign born but US college (In that order although I'd have a hard time arguing against TD being the GOAT):
Olajuwon - "Dream" Nuff said
Duncan - Amazing franchise carying player, greatest PF of all time, arguably top 10 center of all time.
Nash - What can you say about this guy that hasn't been said already? This guy was in the top 5 OVERALL dicussion for a few years. Nash was a top 5 talent without any of the physical gifts that the other guys in this discussion enjoy (size, speed, jumping ability, etc...). Granted he did not stay at this level for very long, still an amazing acomplishment for guy like him.
Ewing - One of the greatest centers of all time despite never winning an NBA title. Size, speed, mobility and great fundamentals for a big man. This guy just had the misfortune of comming into the league durring MJ's dominance and never really have the talent around him to compete.
Foreign born and no US college
Sabonis - NO CONTEST, anyone who saw this guy play in his prime knows this guy was a top 5 to 10 center off all time. Not one single player in this catagory could dominate BOTH ends of the court. Only a tiny fraction of foreign players know how to spell defense yet this guy intimidated guys like David Robinson. Just imagine Shaq's power, Ewings mobility and footwork and TD's shooting range, that was Sabonis. IMO nobody comes close.
Dirk - Amazing talent, matchup nightmare, solid rebounder and practically unlimited offensively. One of the rarest combo's of size and skill ever seen in the NBA.
Manu - Excellent offensive talent, very smart player and a very good closer offensively. The guy is very difficult to defend because of his unorthodox style of play and physical movement plus he has a ton of savy that routinely crosses over into "drama" territory but unfortunately that is part of the NBA game today.
Drazen Petrovich - Actually more talented and skilled than Manu but he did not really get the chance to shine like Manu because he was burried on the bench early in his NBA carrer. The NBA got a glimpse of his talent in NJ but then the guy died in a car accident or something.
AK - I think we all know about AK.
Sariunus Marcelonous (I have no clue how to spell his name) - Played alot like Drazen but with better ball control and size. He did well in GS (I think) under Nelson. Very much a gunner.
Tony Kucok (SIC?) - I think we all know about his talent.
Oscar Schmidt - One of the greatest, if not THE greatest, b-ball shooters to ever live. He couldn't gaurd me on a basketball court but there has never been as prolific a jumpshooter in the history of the foreign game, there are only a handfull of jumpshooters in all NBA history that could match this guy shot for shot.
Overall I'd rank them like this:
Olajuwon
Duncan
Sabonis
Nash
Ewing
Dirk
Manu
Drazen Petrovich
AK
Sariunus Marcelonous
Tony Kucok
Oscar Schmidt
That's all I can think of off the top of my head.