All IMO contestants who have won Fields Medals are the best contestants:
Gregory Margulis (RUS) (Fields Medal 1978, IMO 1959 silver)
Vladimir Drinfeld (RUS) (Fields Medal 1990, IMO 1969 gold)
Jean Christophe Yoccoz (FRA) (Field Medals 1994, IMO 1974 gold)
Richard Borcherds (UNK) (Field Medals 1998, IMO 1977 silver,1978 gold)
Timothy Gowers (UNK) (Field Medals 1998, IMO 1981 gold)
Laurant Lafforgue (FRA) (Fields Medal 2002, IMO 1984,1985 silver)