Peerless as far as Difficulty Goes..... Well Depends On your template, Resources, and Your Tactics.
Personally I'd Say as Far as Hardest To Easiest Peerless BOSS.
1. Shimmering Effusion (It can Get Ugly If you Don't Kill his Spawn)
2. Paroxymous (1 Hit kills! Even at 150 Str, 45+ DCI, 120 Parry he still Can Swat You like a Fly)
3. Grizzle ( Slows Your casting, Eats your armor, Single Tamer can still take him...... )
4. Dreadhorn ( That Curse, Plus a few combo Spells, and A Decent hit, A Really good Dexxer Can solo him)
5. Travesty ( Hit her On the Head She changes Into Something Else! ITS LIKE A CARNIVAL!)
6. Mel ( No Comment, Personally Travesty Is Easier To me, But Thats because I Built my guys to Kick it around)
Hardest To easiest As Far As Getting the Keys.
1. Shimmering Effusion ( Screw Those Hydras, And Unlike the Rest of them You have to match the KEYS AT THE END)
2. Paroxymous (Let The Putrifier Take a Few Swings at you and You'll Understand. 2 Greaters Eat him alive though)
3. Grizzle (Red Death Wanders out while You happen to Be Fighting Michael And Jonathan... ouch......)
4. Dreadhorn (I KEEP KILLING THE CHANGELINGS, AND THEY JUST KEEP COMING BACK!)
5. Travesty ( Disarm, Whirlwind, Chug What More Can I say?)
6. Mel (Seriously? Cmon...... Unless that Crazy Hydra Wanders Over or your A complete NUB You should Never Die getting the keys here)
Funny That went in the same order!