Prankster: No Change
Prankster is just too good to be A Rank. It might not be good as other S Rank abilities, but no doubt, it's an amazing. Sure, Sableye is only UU, but tiers reflect usage, not viability. Priority status, recovery, weather, etc, is a godsend on any pokemon. Prankster is one of those rare abilities that works and is useful to any pokemon it's on (even sweepers benefit from priority Substitute). And thus, because of its usefulness, it deserves S Rank.
Contrary: Change to A Tier
Although Contrary would make some pokemon broken beyond belief, it is quite a limited ability. The large majority of pokemon would get nothing out of it, and it might even prove detrimental in some cases. No mistake, it's powerful in the right hands, but a bit limited, which makes it a perfect A Tier ability.