Castaignede, a Heineken Cup final winner with Toulouse in 1996, commented in his column in the Guardian, "I'd make the Scarlets my favourite because of the way they played in the Pool stages and because, like the Irish, they have fewer games in their bodies than the English and French clubs."
"Llanelli Scarlets made the most striking progress through the Pool stages, considering how difficult the opposition was; they eliminated London Irish and Toulouse in fine style, producing some beautiful rugby along the way.
"To me that says they have the wherewithal and the willpower to go all the way.
"The only obstacle, and it's not a small one, is Munster, the defending champions, but I would look at the fact that the Scarlets are playing in that cauldron of a stadium of theirs and put a few quid on them nonetheless."
Castaignede's other Heineken Cup quarter-final forecasts:
London Wasps v Leinster - "Wasps have a slight advantage"
Biarritz Olympique v Northampton Saints - "The least balanced of the four, this is almost mission impossible for Saints"
Leicester Tigers v Stade Français Paris - "I'd go with my patriotic instincts and back Stade to win here as they have done in the past"