I'm a wine professional, living in Bangkok and travelling frequently throughout South East Asia. I'm a huge fan of Laos and Cambodia and off the beaten track travel. Feel free to contact me at: www.facebook.com/mil.elephants
Bangkok is renowned for nightlife and for many this means Khao San Road, however there are many other areas with their own unique and interesting aspects. Here are a few suggestions for places that are almost always busy, and the fun is guaranteed.
It's late at night, you're out with a group of friends, there is a very good chance you are going to end the night at RCA (Royal Crown Avenue). During the day it's an empty strip, but on any night of the week it comes alive with a dozen different clubs, some with multiple rooms, playing a variety of genres. Bottle service is the norm here. Booking a table is sometimes necessary. My favourite club is Slim.
Talad Rot Fai or Railway Market sets up each night on Ratchada Road, behind the Big C Shopping Mall, not far from Thailand Cultural Centre MRT. It's a great place to sit out at night, with a big variety of music, and bars offering food and drinks until late. It's normally packed with locals and the odd expat, so it's a great place to meet people who aren't tourists.
Silom has something for everybody. Although home to a large portion of Bangkok's gay bars, there are many other types of bars and venues. Many of Bangkok's ritziest rooftop bars are located in the area, including Park Society and Lebua Sky Bar. Maggie Choo's is great for late night cocktails and Ce La Vi, once known as Ku De Ta, is about as Hi So as it gets in Bangkok.
Sukhumvit Soi 11 has changed a lot. It used to be populated with many impromptu street bars, often in the form of combo vans, which would set up on the street late at night. Even the sub soi to the west is being shut down, and icons like Cheap Charles and Charlie Browns will close. It still has lots of bars to choose from and some great places to eat, but condos will take over soon. Get there now.
You can't come to Bangkok and not take in a little of what the city has become know for, it's adult side. Soi Cowboy has in some ways become a common tourist destination for all sorts of reasons. It's relatively small, and if you want to hang out with friends and shoot some pool and drink some beer it's perfect. Wander forth down the Soi and there are many other things on offer.