Thipsamai is one of the most famous (and historical) places for Pad Thai in Bangkok. When I went in December the line for a table was about 20 minutes. It moves fast but that also testifies to its popularity.
I liked it, it checked a box off the to-do list, but I wouldn't roll over in ecstasy after tasting it. The ambiance is really cool. English is absolutely no problem.
You can watch the chefs in the kitchen prepping the food through a glass window which is great fun. Order the bottled orange juice (as recommended to me by a Grab driver) - it's very popular and delicious.

https://thipsamai.com/

This is the location I visited: 313-315 Maha Chai Rd, Khwaeng Samran Rat, Khet Phra Nakhon, Krung Thep Maha Nakhon 10200