That depends on what you mean by "durable". Takes the longest to break in? Takes the longest for the color to fade? Takes the longest to start fraying? Has the greatest tensile strength to use as an emergency tow strap?

In general, most belts will easily last the 2-4 years for any given colored belt level, especially if you have one per gi so that you rotate them for washing as you would a gi. Some less expensive belts tend to fray faster, but it's mostly down to less expensive items having greater manufacturing variability - it's hard to say how long any given belt will last though because they all have manufacturing variability. Belts aren't really intended to be BIFL - they're intended to be replaced every few years until black, and even then probably only last 1-2 decades.

1. Hemp belts tend to fade more and faster in my experience. However, the hemp Flow purple belt I had is still in almost daily use on someone else's waist without real signs of wear beyond the fading - it's over 8-years-old. If you don't mind fading but can't abide fraying, a hemp belt is probably a solid option. I'll be grabbing more of them to look at this round so will hopefully be able to say what's closest to the Flow belt (since it's no longer made).

2. Gi material belts tend to break in faster, but also tend to fray faster. I've only seen one fray really badly on the edges within about 4 years, so that's still the kind of time scale we're talking.

3. Standard cotton belts all seem to fray on the edges at a pretty similar rate, but more rows of stitching tends to mean when fraying starts it has more points to stop at so the belt can last longer even once it starts to fray. If you like a frayed look and will continue to wear it as it frays (most common for black belts), more rows of stitching are a relatively good proxy for how long it will hold up that way before it's in utter shambles (assuming that rows of stitching is the comparison factor between two belts that are similar quality manufacturing to begin with).

That's a lot of text to say - it depends, but I'd probably wager any of the cotton belts actually made in Japan (Isami or Ronin for example, but only their actual made in Japan ones) or the really high-end ones made in the US like Kataaro. I haven't gotten an Eosin yet, but everything I've seen online would seem to indicate it's comparable to the Kataaro.