This could be a really broad topic depending on how it's interpreted, and I've very little experience in the hobby compared to most of you here. "Sought after" could imply rarity, highly desirable or both and may refer to discontinued denim, specific cuts, or even entire brands. But if I were allowed to interpret the thread as a discussion about what we each perceive to be highly desirable brands/cuts/fabric, then this would be my little opinion on the five that's desirable (based on personal experience and some reading):
Tanuki Red Cast fabric (chosen on the basis of denim alone): If there is one pair of jeans in my wardrobe that I genuinely miss, it's these. I grew out of them as they were just starting to look nice. If I could find this denim in an appropriate cut today (new and around RRP), I would run and pounce, not walk, to my wallet and drop my money on them immediately. This is a fabric that has a slight purple hue when raw and just after the first wash, giving way to a beautiful sapphire-blue speckly fade with wear. It holds up well to a lot of washing as well while still returning moderate to high contrast fading. My pair seems to show what I think is three-dimensional fading, which highlights shape when those jeans are worn and giving them a lovely form. Tanuki has a sketchy backstory and rather unclear provenance (Kiya of SE says they are owned and run by Denimio, and I've seen that claim in more than one place), but IMHO, this fabric proves they do know their denim. And in my experience of giving two of their jeans a run-in, I'd say their quality is just fine.
Graph Zero 16 oz Herringbone (chosen on basis of denim, cut, details, quality): GZ has epic levels of cool, perhaps reaching or even surpassing the levels of Kapital and FDMTL. I have a lightly faded pair in their 16oz herringbone fabric that is a great slim-straight cut in a fabric that is known to fade spectacularly. Mine is starting to demonstrate some high contrast fading and I'm really encouraged by that example shown in GZ's IG post. Doesn't hurt that they are serious quality in construction and detail, while being a fraction of the price of what Kapital offers. GZ is a fun brand, and you get that vibe from their IG feed. And their cuts are pretty accessible to most body types. It might seem funny to talk about $160 jeans as being "good value for money", but I figured it might hold some water in a forum like this. They are extremely underrated and should get more patronage.
The Flat Head (chosen on basis of denim, details and quality): examples of amazing fades abound at this site itself, and FH appears to be one of those brands that's had their heyday and now forgotten in preference of other brands. But I love the level of detail in their jeans and their burly, durable fabric that holds up really nicely to frequent washes while rewarding with high contrast fading and tate-ochi. From what I've been reading, I get the impression that the founder, Kobayashi-san, is more interested in making products with detail and quality, rather than cost and profit margin. It may have contributed to the downfall of his company (which subsequently needed to be propped up with an injection of capital), however I admire that dedication to the craft, at the expense of everything else that is practical. I outgrew a pair of 1002 that was looking beautiful even after just a short period of wear, and now have a carpenter's style 8005A in my rotation that has quirky triple-needle stitching in both the outseam and inseam, plus an additional side pocket. And of course, that crispy, inky (almost black) denim with visible vertical streaking. The cuts don't necessarily work for everyone, with common complaints about unusable front pockets or really low rises. But they are perfect for me. Amongst all my jeans, nothing flatters my arse better than my Flat Head jeans. They just look that good!
Roy (chosen on basis of denim, details and quality): I feel like it would be impossible to complete a list like this without at least mentioning this legend and his story. In general, his cuts don't work for me, as they are significantly more roomy than I'd like, and those giant front rises just make me look shorter than I already am. Hence there's a pair of these languishing in my wardrobe. But what I can admire however, is the quality of construction, which is the most obsessive I have seen in any pair of jeans I've ever owned. A seriously high stitch-per-inch count with perfect stitchwork and perfect chain-stitching resulting in equally perfect roping. Plus, the black seed denim he uses is proprietary and now extinct. It just so happens it also fades extremely well, just very slowly. However I think the allure of his brand and products is really from the man himself. When we think, or imagine, those jeans being made by his hands from scratch, and his hands only, with those amazing vintage machines in a dilapidated warehouse, that's when they become even more mythical. His jeans really could be collectibles amongst raw denim aficionados. Today, if you want jeans made by his machines, you gotta look at $800 Buaisou pants. I wouldn't call Roy a jeansmaker. More accurately, he is a master craftsman who just happens to make clothes.
APC (chosen on the basis of denim and cut): their cuts won't work for everyone, but for me, they are just about perfect. Their New Standard is a really great cut for peeps who are built slim and lanky, but they are really quite famous for their Petit Standard as well which is even narrower and low-rise. Some of APC's biggest fans have been bold enough to state "nothing fades quite like APC", said in the tone of "APC fades the best". That's a debatable claim in my book. But proof is in the pudding. And there's plenty of fades at this forum to show it. APC does fade fast, and in an attractive fashion. Their cuts are a French kiss...if you're of a certain body type. I have a New Standard and a Petit Standard, both taken off the local marketplace for less than $40 all up. My wife even noticed the PS the day I donned them. "Nice jeans", she says. Can't say she's ever said that about any of my other jeans.