Definitely Platinum/DP, then LeafGreen/Firered and then Crystal.
I liked Yellow a lot when I was a kid, but I see it now as slightly dull, compared to the remakes. And I played Sapphire the month the first Hoenn games were released, but Sapphire did not amaze me....
I cannot wait for the NA release of HG/SS, they will probably top the list, and I'm really anxious about Geneation V (Roroark looks stupid to me....) and I hope that the V Gen is interesting with new features.