I think I remember hearing back in the day that Blueberry and Strawberry were the highest selling flavours, being trailed by Grape and Lime. Tangerine was the worst selling flavour - back when the 5 flavoured iMacs came out, Apple sold them to dealers in packs of 5, which had to be one of each flavour, so the dealers would often have Tangerine models build up that they couldn't move at RRP, so they were often discounted. Later on though, thankfully they changed that, and allowed dealers to order flavours according to demand.

But yeah, if anything, I'd imagine Lime and Tangerine would be the hardest to find.