For me it has to be the Feast.
That ice cream is just utterly flawless, a design classic.
Don't get me wrong, I love Magnums too, particularly the white chocolate variant, but they're jut not quite as good.
That said, does anyone remember the limited edition '7 Deadly Sins' Magnums from either summer 2002 or 2003?
One of them was tirimisu!
Imagine that, a Magnum with a tirimisu filling? Amazing.