Don't know about best. Cougar (or do they call it coogar?) is just an Air Products brand name for an argon mix welding gas. You can find a whole table of compositions here.
It Varies Neil, one of the best i have tried is ferromaxx, but you tend to get more benifits when teamed up with different wires etc, and welding nice clean metal.
Cleaner, less spatter, less fumes....
Coogar 5 is a good all rounder for mild steel work though.
I'm not sure that either will be "better" or "worse" than the other. The composition of both is similar so I would suggest it really comes down to price/convenience.
Another option if you are a low volume user is adamsgas.co.uk. I've not tried them yet but I understand they don't charge bottle rental and can therefore be quite cost effective. I could be wrong mind, but I'm sure someone more experienced/knowledgable will put us right if I'm incorrect.