Maybe think about Joel Robuchon for lunch, some outrageously good value there if you don't go silly with drinks.

I like nahm a lot and while the restaurant layout isn't terrible for single diners I'm not sure if they could do the set menu for one person. I'd send them an email and ask though as it is the best higher end thai I've had in bangkok

Gaggan is the other one if you're talking famous/fine dining, haven't been there yet but I believe in one section they have bar like seating. You do need to book well in advance though so if you're interested you should try to get a booking asap