Direct from the books, flinds (MM3 62) have the highest average Strength scores with 17. Monster Manual orcs have Strength scores of 17, but that's due to their arrays. Orcs only really get +4 to their Strength scores while flinds get +6.
To win trivia contests, and because sometimes size is more important than raw strength, there's but one Large humanoid, the quinametin (Dragon #317 65), but the average quinametin Strength score is only 15, getting them +4 to their Strength score.
While the greensnake shinomen naga (Medium) remained humanoid, the asp (Large), chameleon (Large), cobra (Large), and constrictor (Huge) shinomen naga (OA 172-5) became monstrous humanoids instead of humanoids in Dungeons and Dragons 3.5 with the article "Oriental Adventures: Eastern Flavor" (Dragon #318 32-48), therefore excluding your best choices.