These may be the worst series to compare as wholes. Simpsons, Futurama, and Family Guy all became worse as time went on and South Park's very concept changed drastically. I guess if I had to..
God Tier
Original Run Family Guy
Simpsons season 3-10
Current South Park (the change was gradual so I'm not going to sit down and quantify it)
Decent Overall
Original Run Family Guy
Some good some bad
Current Futurama
Seasons 11+Simpsons
Pretty crappy
Early South Park
Current Family Guy
Simpsons 1-2
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