I don't think there is such a thing as "best." Merely different types and styles. The one that meets your needs is the best one for you.
That said, I have used all sorts, from the American Harvest to the Excaliber. And I've settled on the American Harvest. In fact, when mine finally gave up the ghost after 12 hard years I replaced it with another.
These are the bottom-fan models (there are reasons they are preferred). My replacement was the same model number, except they had improved it, and it went from 8 tray capacity to 12. Certainly not a problem.
Unfortunatley, Nesco is moving away from the bottom fan versions. There is only one model like that now, with a 30 (that is not a typo) tray capacity---and a pricetag to match. The rest are top-fan models.
When chooseing one, some things to consider:
1. Tray movement. With many machines you have to shift the trays around, to assure even drying. That means developing a timing and rhythym so that everything becomes the bottom shelf for equal amounts of time.
2. Capacity. Can you add additional trays? This might not seem a big deal, at first. But drying food is addictive, and the more you do it the more you want to do it. From a time & cost basis, it's better to do one batch of 6 trays than two batches of 3 trays each.
3. Flexibility. Can you vary the spacing between trays? If not, you are stuck with one size of food, or else, as with the Am. Harvest, will have to modify the trays themself.
4. Tray configuration. Are only standard trays available? Or are there other formats---with smaller or larger holes, for instance, or solid to make fruit leathers---available. Note: Merely having them available is not a reason for deciding on a make/model. If you never make fruit leather, who cares if they offer a tray for it?
Let me add, too, that thermostatic control is the second best thing to useless on a dehydrator. Forget instructions to the contrariy---low and slow is the way to proceed. I never work at more than 115F. And the fact is, much over 145F and you're in danger of cooking the food rather than drying it. There are other permanent changes the higher you go. At 165F, for instance, the volatile oils (including capcaicin) in chilies cook off, and they wind up tastless.
The guidelines provided for what temperature to use for what food type are to protect the manufacturer from liability suits. But they are not the best settings for a quality product.