Blemishes may or may not be a problem (if you're buying organic you'll see more blemishes). Look for unpunctured skin and firmness with good color. I prefer Granny Smith. For me, the harder the better and I like mine cold.
For health's sake, though, make sure the skin is not cut in anyway.
I love our locally grown Macintosh - great baking apples that have a wonderful smell. I also like granny smith apples for baking. I look for firm apples and then I smell them. I also like my apples cold.
What kind of apples are available in Singapore? I look for firm, non-blemished with no signs of worms (no holes in them). I won't buy them if they were waxed (makes them shiny for appearance sake and I have to wash off the wax).