I was at Sepang two years ago. On the Friday I went onto the main concourse, trying the main straight, the back straight and the corner between. Of those the stand on the back straight was best. For the race it was stand K1, you get a good view of the start, that long straight turn one and the entrance to turn two.
The down bits about Sepang are that it is a longish journey from KL (although there is a good choice of transport), the circuit surrounds become a sea of mud when it rains (as it frequently does over the course of the weekend), you are not allowed to take food and/or drink into the circuit (putting it in the bottom of a deep rucksack and going in when they're busy worked for me) and when it does rain it comes down in bucketfulls, you then need to be further back than row F.
The upsides are that it is usually an interesting race and it is located in Malaysia.