Over the years though, as most of my flats fishing has been in Japan and the flats here tend to be fairly diverse in terms of what you'll encounter, and as a result, I've really moved towards having a few multipurpose patterns in my flats boxes most of the time. On my recent trip I was initially carrying a lot of heavy crabs because I was expecting mostly triggers in a high current environment. However, I didn't really find many (any) so I had to fall back on a generic pattern because I was seeing assorted trevally, emperor, snapper and others. I chose a squimp. Although it's often called the bonefish squimp, it's such a versatile flats creature that I never think of it as a bonefish specific pattern. I definitely think it's an underrated fly because of the bonefish fly label, it's not new and fancy and it's traditionally tied with a basic chenille body.
But for me, it's about as close to a perfect flats fly as they come. First, everything eats it, everything. Even species that shouldn't, will. It is a pretty easy non-fouling tie that lends itself to size, weight & material adjustments. Even a fairly large squimp can be tied quite lightly weighted and will reliably fish the right way up and track when being stripped. The craft-fur can be replaced with just about any synthetic or natural hair to suit size, colour or mobility requirements - a wee one tied with rabbit is great if you need a fly that is still full of movement while sitting static on the bottom. The rayon chenille body can likewise be replaced with diamond braid, dubbing or something else you fancy. I like a rough dubbing body, but that's largely down to confidence. Not to say the old fashioned chenille doesn't work, it does but it looks a bit rubbish when compared with other options.
But for me the best thing about it is that you can change how you fish it pretty drastically and it still remains effective. This works whether it's mid-retrieve and you're trying to feed a reluctant fish or you're encountering different species in one area. It's a great fly for fast moving, fast feeding species like trevally you can pop a bit of speed in the retrieve to get their attention then watch them light up and get excited as they chase it down and eat it. Or maybe the fish is a bit neutral and you need to slow it down and let it dive to the coral and "hide" in order to get the fish to eat. But the next fish you see could be a trigger or a spangled emperor and that fast pull is a non starter so you let it sit, give it a bump watch the fish and base your retrieve on its reaction. The front legs really come into their own in keeping that fly alive in these situations.
If I had to pick one fly for the flats, it might well be a squimp. There are other shrimpy flies you can put in your box, but few of them are as easy to get right, versatile and as effective as a squimp.