My family and I have known Niza for over seven years now (since we moved to Valladolid), and it's undoubtedly the restaurant we've visited most in that time. It's a restaurant that excels in every way: the service is professional while still managing to feel friendly (and even family-friendly; every time you go to Niza, you get a great atmosphere, largely thanks to the wonderful staff), and the food is superbly prepared, with quality ingredients.
Personally, as a person with celiac disease, it's one of those few places that isn't completely gluten-free, but you still feel cared for and looked after. It's a restaurant where those of us who can't eat gluten have a wide variety of dishes (I'd even say most of the menu is or could be gluten-free) and is completely safe. They also have the best gluten-free pizza dough I've ever tried, and I highly recommend ordering it for anyone with celiac disease so they can enjoy a good pizza. The gluten-free desserts are also excellent, and while they don't have as many options as the rest of the menu, there are a few, and the dessert of the day is usually gluten-free. My favorite after a heavy meal is the tiramisu—I'd say it's the lightest on the menu—but all their cakes are recommended if you have room :)
I'd also like to highlight that they have a considerable number of vegetarian and some vegan options, and their team is very attentive to all allergies.