Speaking about Thai brands, I discovered by chance last year "Arrrow" (Excellency line) in Robinson as well and it has become my favorite brand for shirts. I came back 2 weeks ago and bought 3 shirts from this brand again. Good cotton weaving quality and great value. I bought last year 1 shirt for ~30 Euros which I found again at "Galeries Lafayette" in Paris for 75 Euros (not sales).
I bought a nice white "SFARE" shirt made in Thailand for one of my friend. He was delighted and always wear it for a night out.
Some known brand names like "Ungaro" or "Daniel Hechter" can be found for much cheaper than in Europe in Robinson, Central department store or Emporium department store. I love those places! And you can get 7% VAT refund at the airport if you buy for more than 2kTHB per day / 5kTHB in total.
Take a look in the shop in the back (ground floor) of Ploenchit Center (Suk Soi 2). Sometimes bargains on designer clothing.... Valentino jeans THB 1,200 or so.
