We tried a few stalls, but our favourite was a stall across the road from Vie Hotel, Bangkok where we were staying. You can easily find him as he is located under Ratchathewi BTS, next to Asia Hotel.
There is a wide range of beverages you can order, Thai coffee, tea, Nescafé, mocha etc. We noticed he only prepares iced drinks. Didn't ask him why because we can't speak Thai. Must be the weather was too hot for hot drinks. Duh!!
You can't miss him as there will be people queuing. So, be prepare to wait a while for your turn.
Price : 20 baht per drink, very cheap if you ask me.