I don't think it is any big secret that one of the best views of Kuala Lumpur's Petronas Towers can be seen at the Sky Bar. The Sky Bar is located on the 33rd floor of the Traders Hotel. It is über hip, with an amazing interior, but its coolness doesn't warrant the prices to eat there (considering you can have a fabulous meal in K.L. for $5-10), so I would recommend just going for a drink (or three, in my case).
No reservation is needed to grab a table near the bar, and, if you're lucky enough, you can get a high-top table near the windows. Have a few drinks while ambient lounge music pumps from the speakers and have fun people watching.