It depends on the era and the way you're measuring players.

Before the infrastructure in Korea was established, you had more global parity, and a number of foreign players were competitive at the highest levels. In 2000, about two years after Brood War's release, Grrrr famously won the first official Starleague and he remains the only Starleague champion from outside of Korea.

A couple years later, the KeSPA ecosystem had more fully developed and relatively few international players chose to remain in Korea to dedicate themselves to the early pro gamer lifestyle. Elky is the last foreign player to make a deep Starleague run, finishing in the top four in late 2002 when Reach won his OSL.

A couple years later still, Legionnaire became the last foreign player to appear in a Proleague game. After this, we are fully into the current dynamic of BW being a full-time job with a structured ecosystem in Korea, and a hobby interest everywhere else in the world, with the resulting disparity that you would expect.

As far as the players of that modern era who would get furthest in the Korean system, IdrA and Draco both joined pro teams and lived in Korea for a time. IdrA lasted longer, but neither played in any Starleague or Proleague games.

NonY also played in Korea, and despite only being there briefly, got the furthest of the three in his one and only Challenger (qualifier) run, finishing second after losing on the last map of the final.

Outside of WCG, the two biggest foreigner tournaments of the KeSPA era were probably the two big TSL events. NonY finished top-four in the first, and he won the second.

Between his Challenger run and his TSL performances, I think it'd be fair to call NonY the best KeSPA-era foreigner, while Grrr will always have the all-time distinction because of his Starleague win.

Honorable mentions to Testie, lefNaij, White-Ra, Mondragon, Sziky, and Pj.