Okay, so these may not be someone else's top 5, but for me these are the Malls I recommend to friends and family who come to visit. My rule of thumb is that the mall has something out of the ordinary from most western shopping malls.
Bangkok is awash with Malls, and the more modern they are the more alike they tend to be. They'll generally have a similar layout, a similar range of stores and many of those will be internationally recognised brands, making the shopping experience less than unique.
These also tend to be the malls closest to where the BTS (monorail) or MRT (underground) runs and often the most expensive of Malls. Generally speaking designer and known brands are more expensive in Thailand than in the west due to shockingly high import duties, so find a bargain generally means finding something that's made in Thailand.
Jatujak (Chatuchak) Market, Jatujak
Only open on weekends (other than plants, animals, furniture and kitchenware) this is the big daddy of markets in Bangkok. It's easily reached by BTS (Mo Chit Station) or MRT (Kampeang Phet) and offers a witheringly huge range of shopping options. It's actually a complex of stalls and buildings that sell everything from massive sculptures and furniture to underwear and designer clothing. It's estimated that there are 8000 stalls.
Fortune Mall, Rama 9
This is my go-to electronics mall. Most often visitors to Bangkok go to Pantip Plaza or MBK for their electronic needs, but Fortune Mall offers a much better experience. Firstly this not a Mall with a large tourist clientele and prices reflect this. If you wander from one shop to the next pricing products you'll find that there's very little variance and this is reflective of the true market price. All the major brands are represented, and there are plenty of stalls selling a very large range of add ons and gadgets. Additionally, and unlike some other malls there are several specialist shops (musical instruments, electrical components). MRT Rama 9 station.
Union Mall
This is where all my Thai female friends go to shop for clothes. 8 floors of small owner occupied stalls selling predominantly female clothes with infinite variety. There are over 1200 stalls selling all manner of clothing, footwear, accessories and much more. Most of these are made locally in some Bangkok's clothing districts and often are made by the owner of the stall. Bargains can be found but generally you'll need to buy several or multiple items to get a discount. You'll often see people buying large amounts to take back to their hometown to sell. MRT Phahol Yothin
Talad Rot Fai
Talad Rot Fai or Railway Market is something of a franchise, the original market has since closed and a handful of new markets have opened up in their place. The most readily accessible and the best is on Ratchada Road and offers not only clothing and gifts similar to many other markets it has many streets bars and restaurants offering great music and atmosphere until very late at night. The market is behind Big C on Ratchada Road. MRT Thailand Cultural Centre.
Victory Monument
Although not a mall as such, more a precinct, the area around Victory Monument is jam packed with small malls and street stalls selling mostly clothing. The area is one the city's major transit hubs and is busy at almost any time. Fashion Mall, Century Mall and Centre One are three of the malls in the area, each with a slightly different mix of shops. There's also plenty of great street food if you get peckish. BTS Victory Monument
