Basic, Mackerel, smoked & peppered 100g, EE -1 syn / Original - 1 syn / Green - 15.5 syns.
There's quite a few to choose from so it might depend on where you get them, the co-op ones come out at 1.5 syns, the Waitrose love life come out at 0.5 syns per 120g serving and there's a few other, can't find any free ones though.