Heracross has always been the Pokemon I've been confused about. It beat what, Scizor, Magmar and Kricketune? None of those wins are that notable, and definitely not enough to put him "up there". Sure, it damaged Darkrai, but so did Gible! Heracross is strong, but not strong enough to count as one of Ash's strongest.
The first five are obvious, not really sure about the sixth though I've always liked to think of Glalie as one of his strongest. It doesn't really compare to Charizard and co. but it was a beast in the Hoenn league. Swellow could possibly take the sixth spot too.