As already said, Starrett pretty much sets the standard. However, there are plenty of other brands which are perfectly acceptable for use, even the lower priced ones.
Personally, I detest plastic slides. Don't like the idea of it in a caliper. I don't care whether it's digital or analog, so long as it's accurate AND repeatable.
You can test accuracy and repeatability using a set of feeler gages. I suppose if you wished, you could also invest in a set of calibration rods or discs. Actually, you don't need a set...just one that's close to the range you would use, such as a one inch one for handgun ammo. You can google this and find places to buy them from. Expect to pay about 50 bones or so for one this size.
I have an OEM digital slide caliper I picked up at an auto parts store for about $25 or so, if I remember correctly. It's accurate and repeatable.
Another thing to look for is ruler markings on the slide. Mine has them in both metric and English and the difference between 1.062 and 1.019 is quite visibly noticable. Just another "attention to detail" feature you can use to easily double check for gross errors.
It should go without saying that calipers, being a fairly delicate precision instrument, should be cared for accordingly. Get one with it's own case and be sure to store it in the case when not in use. Change the batteries periodically, remove them when not used long term, make sure they're kept clean and dry, make sure they operate smoothly, and check their calibration periodically against feeler gages or calibration rods. And don't drop them.
Even so, like any such precision tool, sometimes you need to replace them for whatever reason. Your calibration checks will tell you when this is necessary.