Gaara
Mei
Onoki
Ay
Tsunade
-> Tsunade is the weakest gokage because she's support mainly. Sure she can't die, but she still can't kill most people
-> Ay's next because he's not that versatile. Speed is fine and all, but the second someone has good defence (Bone release, sand armor, susano'o, V2 cloak, Raiton Shroud, etc) his speed is pretty useless as his strength isn't that astounding
-> Then comes onoki. He doesn't have much stamina and his back always goes it if he tries to do quick highspeed movements or agile feats. His jinton is 1 shot and all but it takes time and can be dodged. Furthermore he has to be stationary to use it making him open to counter attack.
-> Mei's next because she's bamf AF. She's the most versatile Kage and can literally take on any opponent. Sure, She can't win everyone(based on current feats), but she has skills to counter near any opponent, even the visually gifted sharingan users and strong bodied Senjus.
-> Gaara comes first because with sand he can do anything, and as long as you're not fighting in the middle of the ocean, he'd have sand. For that matter, unless you're over the mariana trench, he'd probably still be able to bring up the sand from the bottom of the ocean...
As to your last point about gaara Vs deidara. Gaara can never ever beat deid. There's no scenario that'd work because deid is just a horrid match up for him. Deidara came unprepared, in a location that was greatly to gaara's advantage, while gaara had shukaku and with the intent to capture instead of kill yet still won.