S Rank:
Rook, Scathe, Ninjette

A Rank:
Scuttlemuck, Leatherhead, Oddilus, Kunak, Dr Jennifer, Ace

B Rank:
Sprout, Kovax, Xavilla, Dr Trip, Blitz, Platimus, Vooghoul, Plan-Kan, Z-314,

C Rank:
Mainframe, Melody, Kilowatt, Brutus, Nexi, Zert, Minos

D Rank:
Oozelo

I think we can all agree about the candidates in S Tier:
Scathe with his high damage and sustain, not to mention he's an anti-support with his heal reduction and knockdown.
Rook with her movement options and single-target damage with her 1st ability
and Ninjette with her speed and overall damage output (also having a 100% crit cahnce out of stealth)

For A Rank:
I placed Scuttlemuck at the top of A rank due to his sheer utility he brings to the table, I mean he has a line knockdown/knockback and team-healing properties within his 1st and 3rd ability.
Leatherhead was an easy pick for A-Rank thanks to his tankyness and utility, nuff said.
Oddilus was a different story however, as his damage as a brawler came from his ablities being used to a masterful point, but when pulled off, he is truly powerful.
Kunak is one of those champs that will do a lot of damage without much skill, I mean just basic attack and use your 3rd ability and things just blow up before you with his trait giving him free crits.
Dr Jennifer is a whole different story from Kunak, ol' Jenny requires SO much mechanical skill and she can be built to be pretty much anything, Heal supp, Tanky disruptor, you name it!
Finally we have Ace, who has increadible movement options and his Area of Effect CC (Crowd Control a.k.a Any kind of Hex/Debuff)

For B Rank:
Kovax is a special case because i had no idea where to put him on the list, after playing a few games with him and trying out every scenario, i can safely put him in B-Rank thanks to his damage over time and his ability to burst people down. (Same goes to Xavilla but her burst is pretty hard to pull off).
Dr Trip was an easy one to place, as he has an increadible movement option with his borrow, his splat potion dealing Aoe Damge with a Blind (and god knows how crippling those things are) and his healing balm.
Blitz was supposed to be in A Rank but I retought about everything and eventually put him back down to B Rank, He may have high damage but he was supposed to be a support and his utility options aren't great.
Platimus and Sprout are on the list for the same reasons because they practically do the same thing with the exception of Sprout being able to place turrets.

Do I need say anything for C Rank...?

But for D Rank however, I feel that I will get a few complaints over my choice,
here we have Oozelo, sure he may have a heal, but its indirect meaning if the enemy team is smart, they can prevent any healing from occuring, also he has an ability which is a huge AoE DoT, this wouldn't be so bad if it actually did any damage and couldn't easily be broken by a knockback or stun (which half the cast own in some way, shape or form). It just ruins this champ for me for those reasons.

I hope i managed to not p*** too many people off, if you have opinions of your own, please post them in the comments below. Also please don't judge me upon my choices as they are just my opinion afterall.