The jury is still out on which is the *best* Pad Thai in Bangkok, as far as my Bangkok relatives are concerned. One aunt insisted the one off Yaowarat Rd which dishes out miniscule plates of Pad Thai, simply-fried with eggs, beansprouts, tofu and shrimps, is the best. Yet, another aunt and her husband only go to the one at Rama III Rd's night street stalls. Out of convenience, I'd go to either S&P or onbeof the other mall-located chains for my Pad Thai fix.
But a majority of Bangkokians I know would point to the outrageously popular Thip Samai on Samran Rat as the best in town. Starting from a simple streetside stall, cooking with a simple wok on one charcoal burner, it's morphed, 5 decades later, into a restaurant proper with its own website (http://www.thipsamai.com/), but still cooks its Pad Thai using charcoal stoves.
Be forewarned though: Thip Samai's Pad Thai is the "wetter" version. I normally liked the drier renditions in other places, but Thip Samai's somehow grows on one, and the standards have been consistently high over the past 3-4 decades I'd been eating there.
Thip Samai started back on 9 Sep 1966: any place that's been around for close to half a century - and still packing in the crowds - is definitely doing something right.
Thip Samai (ผัดไทยทิพย์สมัย).
313 Maha Chai Road
Samran Rat, Phra Nakhon
Tel: +662 221 6280
Address in Thai: 313 ถ.มหาไชย สำราษราษฎร์ พระนคร กรุงเทพฯ 10200