1. Onoki
2. A
3. Gaara
4. Tsunade
5. Mei
My Explanation: Onoki has a homing jutsu and the ability to fly makes it impossible for Special enemies to hit him.
the fact that he had a Long life until now makes him also smart and come up with good strategies in fights I suppose.
A is a fast. strong lightning user which can Focus his chakra and do high accurated attacks. The fact that hes some Kind of coldblooded makes him a brutal killer.
Gaara - I got to say that if he gets the shukaku back he will probably go up to rank 1 or maybe 2 in Terms of strenght.
The fact that he is so Young and went pure solo in the past makes me think about strategies but his perfect defense is
powerful. Still we had to see how chidori went through it and Naruto once in a movie broke it with a rasengan too.
We better call it "one of the best defenses"
Tsunade : With her pure strength, her high Age and the fact that you cant kill her (at high chakra cost tho) makes her
an hard to fight Opponent. The fact that she only has high taijutsu and isnt so good in Terms of movement Speed makes her only take place 4 for me.
Mei : Mei is Young and seems to have weird characteristics. Her jutsus are good, but does she have enough chakra to spam them ? What is she really capable of ? The fact that we havent seen much of her and no real Spotlight shined on her makes me think that kishimoto totally forgot her or doesnt want to Show her all the way. this is the reason why i think she is only place 5.