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