The MSW name has changed over time. What started out as Gua Musang (a district off Southern Kelantan), slowly developed into other names, before it adopts the original name, but in chinese words (not the same meaning though), Mao Shan Wang.
MSW must have gotten popular sometime in the mid 2000s. I first tried it sometime in 2002/3 period, in Kuala Lumpur Malaysia. Back then, who knows about MSW? It was unheard off in Singapore. I was first recommended this fruit, as Huang Jiang (Chinese word, Yellow Ginger).
How did Huang Jiang came about? Definitely, this name won't be as catchy as Mao Shan Wang (Cat Mountain King). Look at the color, yellow as old ginger. Taste wise, it has that 'spiciness' of old ginger.
Ok but than, that is not the original name after all. If I'm not mistaken, the fruit originate from Gua Musang (a town in Kelantan, Malaysia). So to some Malaysian, this durian is also called Gua Musang or Musang King (after the town). Some Malaysia durians' name has a story behind, this is one of them.
A little more...
MSW is a very popular durian variety in Singapore, more popular than D24 in the last few years (perhaps around later part of 2000s). The strong character and taste got people hooked. There are a few names for this durian, including e usual MSW, to Wang Zhong Wang (king of kings), etc.
There is this brown 'Star' lines that runs along the split lines, from the bottom upwards (not 100% has this, but it's a good indication). The thorns are usually very rough and hard. Shell color can be healthy green or green with shades of dirty/dirt brown color. I prefer those that looks a bit dirt brown.
Typically
Often, the whole durian is slightly odd shape, either bigger bottom or sometimes a bit slant shape (but huge). It would average around 2 - 3 kg (there are larger ones weighing 3-4 kg).
I tends to go for a mid-large size MSW, weigh above 1.5kg. The larger, the more value for money. How so?
A smaller MSW, say, below 1.5kg, usually has lesser seeds. So, it's not that the seller is selling you lesser, but that's how this variant of durian is. Larger MSW, above 2kg preferred, usually has more flesh/seeds. Even if means paying more (since it is by weight), you are actually going to get more flesh. Give this example.
A 3kg MSW with 5 full sides will produce a lot of flesh/seeds. A small 1.5kg MSW usually has lesser sides, lesser flesh/seeds. Even if you buy 2 fruits of 1.5kg each, the total flesh/seeds will be lesser than 1 fruit weighing 3kg. So if I'm going for quantity, I rather pay for a large fruit. But if I am willing to go for lesser quantity, usually quality can be higher.
Texture, Taste, Aroma (Flesh)
It has to be evenly deep/mature yellow. If there is shades of grey, it is not a good MSW.
Another hint on the color. Darker shades of yellow tends to look glossy and will taste very strong, gingery spicy and bitter (sometimes with a hint of coffee aroma too). Lighter shades tends to look 'plasticky' shinny (different from wet glossy look) and will taste lighter and tends to be sweet. Both colors has it's own followers (some likes it bitter, some likes it sweet).
The flesh is usually very thick, while the seeds are often flat (maybe 60-80% of the time). Sometimes, the seeds are exposed, sometimes the flesh has a 'burnt' corner. These 2 signs are ok, no need to fuss about.
Reason why many people swear by MSW is the strong character, yet some doesn't like it as it gets too strong. So yes, taste test, good MSW will hit the nerves. It will be inaccurate to say MSW is mostly bitter or sweet. The color/shade test above, will give you the indication of bitterness or sweet.
Check the appearance. Go for odd shaped, as round MSW doesn't appeal to me. It is not about the number of seeds here, but the quality of flesh.
Also, when you take a sniff test, it shd have a distinct smell, yet not overly strong. Overwhelming smell tends to be over ripe (remember?) but this does appeal to me a lot since the taste will be VERY STRONG usually. No need to shake the durian.
Dec 2015 update - Small trick up my sleeve. If you want more seeds/flesh, go for rounder shaped MSW. A irregular shaped one has lesser seeds, but let's also understand that more doesn't necessary means better. Imagine the nutrients going into 1 fruit, divide by number of seeds? Not always true, but well, that's just me.
Hint
As above, the color of the durian flesh will tell you the quality. A deeper color tone will indicate a nicer taste (bitter). Light colored are less strong in taste and tends to be sweet (for those who are not extreme).
Personally
I'm ready to sink my teeth into good MSW anytime. However, like I shared (repeatively), darker/mature yellow for stronger taste, which I prefer. I will prefer 2kg onwards fruit, not small ones.
Show of cards... somewhere around 8/10. (D24 wins a little since it has more personality/character. Personal choice, I choose Tekka over MSW).
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Passing Off As....
In recent years, a lot of durian stalls have been passing off durians source (State) to make it more appealing, and consumers are gullible to believe claims. MSW may have originated from Kelantan, but trust me, none of these durians sold in Singapore originates from Kelantan. So if a stall passed it off as Kelantan's MSW, I will walk away, they cannot be trusted.
(ps : am i not long winded?)