Grammercy Sash Saw is the best. takes a bit to get used to but works pretty fast once u get the hang of it.....
I usually stay out of these discussions, because there is no simple answer to the OP's question. Most folks simply recommend what they like & are used to but that is not a terribly objective way to evaluate anything. Recommending something like the Gramercy sash saw as a dovetailer is getting right out there, imo, but if it works for the poster, then it obviously works!
To bring a bit of objectivity to the discussion, these are things I look for in a dovetail saw - mileages vary, but these features are common to all saws, and are worth considering when contemplating buying a saw.
Size: There is no point in pushing around more saw than you need for the job. A light saw is maneuverable and easy to place, but what you consider 'light' will depend on your size & strength & the job in hand. I certainly wouldn't use a 14 x 3" saw for cutting dovetails on drawers, but I would probably use something similar for cutting the whopping great dovetails on my bench ends. For finer work, I like a saw around 200-225mm long, which will give a decent stroke in stock up to around 30mm thick. Most furniture D/Ts are cut in material around 16-18mm thick.
TPI: For any saw, there is a trade-off between speed & smoothness of cut. I like dovetails to fit nicely off-saw, so I need a very clean & accurate cut. I find that 15-16tpi is the sweet spot between speed & quality of cut for the woods I generally use. For really narrow stock, I prefer 18, and I occasionally use a 24tpi saw for really thin stuff. Your preference is going to vary depending on what woods & what thicknesses you mostly saw, but if I had only one saw for dovetailing, I would go for the 15tpi one as a good all-rounder.
Plate thickness: It is often claimed that thin-plate saws cut faster, which is simply not true, sharpness & tooth size determine speed of cut. All else being equal, a thicker plate of the same pitch will cut at the same speed. It may require a bit more effort because you are removing slightly more wood with each stroke, but the difference between say, 15 thou & 20 thou plate is probably undetectable. Thin plate saws make fine kerfs, which can be desirable, but they are less robust. I usually advise going for 20 thou plate for your first D/T saw.
Grip size & Hang angle: These are features that makes a huge difference to your perception of comfort in a saw. I find that wincy narrow grip on the Gramercy D/T saw not at all to my liking, but plenty of people are happy with it. I also like the grip to be rounded & shaped more.
But the single most important feature for comfortable & accurate sawing in my view is the hang angle of the grip. If you are getting along in life & your eyes ain't wot they uster be, you probably notice you are putting the work up higher so you can see your lines. I cut dovetails in a twin-screw vise that puts the sawing point about 200mm or more above normal bench height. When sawing, with my forearm roughly parallel to the bench the tooth line of the saw is also parallel to the bench, so that it's easy to saw to the line back & front without constant checking. This requres a near-vertical angle to the grip.
If I were to use a saw with a handle like the Gramercy (not picking on you, Mr. Gramercy, I'm only using you as a example that anyone can refer to!
), my hand would have to be twisted at a very uncomfortable angle (something else that comes with age, I'm not as flexible as I once was!) in order to keep the tooth line horizontal.
Here are a couple of dovetail saws:
The saw on the left is a copy of the small Kenyon saw from the Seaton tool chest. The hang angle may be a degree or so lower than the original, but it's close enough. This is quite a bit more 'vertical' than the grip of the Gramercy, but I've found a lot of people like this handle (mostly younger ones with good eyesight who work a bit lower down). The handle on the right has a rather extreme angle, but it's what I like because even with specs, I need the work up high and this gives me a comfortable wrist position when the tooth line of the saw is horizontal.
All of the above will vary with individual tastes, woods used, & work styles, but at least they are objective features to go by when evaluating a saw. For my money, I'd say look for an old saw & learn to fettle it, but that's not for everyone. It does take time & practice to become truy competent at saw sharpening, but most people of normal ability can do a reasonable job after a few tries, and even a not-so-well sharpened saw almost always cuts better than a dull one!
Cheers,