Office district with many condominiums as well, nice food and places to go out: Sukhumvit is the place to be!
Sukhumvit is maybe the most well known street in Bangkok and many try their best to find a condo in this area. It is composed of many sois (or small streets departing from Sukhumvit) with some popular names as well such as Asoke, Thonglor, Ekamai and On Nut.
Sukhumvit is shared between condominiums (high rise and low rise), office buildings, hotels and all modern facilities. This explains certainly why a lot of non thai national appreciates that area despite the heavy traffic with huge traffic jams on peak times (early morning and late afternoon, linked to office hours).
A lot of westerners decide to settle along sukhumvit, and in particular from sukhumvit soi 0 (Phloen Chit BTS station) to sukhumvit soi 32 (around BTS Phrom Phong). There will be mainly european and american people living in there, commuting from home to the nearest BTS / MRT station in order to join their offices. There are actually also buildings that are sharing condominium units, offices and even shopping area so you won't even need to go on the street.
A small korean community is located as well early sukhumvit between soi 12 and 16 with a small "korean town" at the entrance of soi 12.
Japanese people will tend more to live from Asoke area (Sukhumvit soi 21) to Thonglor (soi 55). There is large japanese community around the Emporium department store with many japanese restaurants and shopping area (soi 24, 33/1 and nearby area).
One very trendy location popular among thais and foreigners is Thonglor (sukhumvit soi 55): this is a large residential area mainly, with plenty of shopping areas around and modern construction (mostly luxurious or high quality condominiums). You may want to spend some time around there and the nearby connected soi 63, Ekamai, to find a cozy place.
On Nut is also quite popular as it was previously the last BTS station of the Sukhumvit BTS line. The street is also connected to Ramkhamhaeng street making it popular for thai as there are some large university campus on that road. However the BTS is now extended to Bangna. A lot of new condominiums projects are rising there and we will certainly see new popular places there in a few years!
One of the weakest point of Sukhumvit road is the very dense traffic that you will find there all day, but more particularly in early morning and late afternoon as offices open or close. However do not give up right now on the area! This is also the best deserved by the BTS (elevated train) with not less than 12 stations along the road! The MRT (subway) also is connected to the BTS at Asoke intersection. Just be aware those can also be packed when people commute to go to work so you may want to check your nearby station at the peak time to validate your choice of condo.
Along sukhumvit you will have no problem finding all shopping that you need either for your cooking or for your weekly fashion hunting! Next to BTS Asoke station there is the newly opened Terminal 21 that provides a large array of items to shop. There is also the very popular Emporium department store a bit further next to BTS Phrom Phong station. You will also find imported items in Villa Market and Foodland. Big C extra (previously Carrefour) also provides some imported and affordable French food.
The most popular hospitals are all located nearby Sukhumvit. Getting a condo near Bumrungrad hospital on Sukhumvit soi 3 is great if you plan to be in Bangkok for medical reasons. Else you can find Samitivej Hospital on soi 49, Bangkok Hospital at the end of soi Thonglor (on Petchaburi Road)... If you are sick or need medical assistance you can choose between the large choice in that area, however please note that those are not the cheapest ones! They are popular also for medical tourism so many foreigners are looking for places to rent located nearby.
If you enjoy nightlife or prefer to stay away from it, you may want to know that there is a large number of bars (and some prostitution as well) around sukhumvit soi 4 (soi nana) and at the beginning of sukhumvit soi 21 (asoke, soi cowboy): those places can be quite noisy at night.
There are also a number of very nice and trendy restaurants everywhere around Sukhumvit road, as well as some bars and wine tasting places. You will surely find some places to hang out having lunch / dinner. Sukhumvit 11 (Tapas Cafe, Limoncello, Baan Rot Fai...) and Thonglor (many great japanese, HOBs...) are some of my favorite places for dining but there are many others to be discovered.
If you are more looking for some party and clubbing you will also get a lot of choice on Sukhumvit road, especially in some sois of Thonglor.