I have flown with all skyteam member airlines except for AM.
My ranking is as follow:

Y class Long-Haul
1. KE(B744,B777,A330)
2. AF(B777, A340)
3. KL(only if you do not fly long haul with b767)
4. NW(only if you fly A330)
5. CZ(Airbus 310 is shabby looking, though)
6. DL
7 AZ


Y class short-haul
1. KE(B744, B737)
2. KL(B737,F70,F50)
3. AF(EMB-145, A320)
4. Cz(B737,Atr-72)
5. AZ(Emb-70)
6. DL(M80)
7. NW(DC-9, I hate this plane)

C class Long haul

1. AF(B777, A340)
2. KE(B777)
3. KL(B747M)
4. NW(A330)
5. AZ(B777)
6. CZ(A310)
7. DL(B777)


Based on meal service, Friendly cabin crew, In-Flight entertainment, condition of the aircraft, punctuality. ground service, in case you miss the connecting flights.