The Spanish refer to smoked paprika as pimenton and they use it in a range of their dishes like paella as well as in various soups and stews. It consists of peppers that are first dried, then smoked and ground to a powder. The varieties of smoked paprika range from sweet to hot and impart a subtle smokiness as well as a touch of umami to dishes.
Smoked paprika should be a staple in your spice cabinet whether you like to cook traditional Spanish dishes or not. Among many other uses, it is an excellent complement to other spices in barbecue rub blends. You can find it in many supermarkets as well as in some specialty food stores.
Your best bet: Chipotle pepper powder
Chipotle pepper powder is an effective substitute because it is smoked. Chipotle powder is used mostly in Mexican and Tex-Mex dishes so you should be able to find it in the Mexican aisle of your grocery store. It works in stews as well as in barbecue rub mixes. It can provide both the smokiness and the deep red color that you want from smoked paprika.
It is important to note that chipotle pepper powder can be much hotter than the hottest smoked paprika. In order to mitigate that heat, you will want to use less of it than the amount that your recipe requires for smoked paprika. If you need more of the red color, consider adding sweet paprika along with it. Use about a third of the amount that the recipe indicates for smoked paprika.
A decent second choice: Sweet paprika and cumin
Sweet paprika provides the red color that you want from paprika, but none of the smoky flavor or heat. Adding a little cumin can provide the missing smokiness. In addition, cumin will pair well with most of the other spices in a dish that requires smoked paprika. If your recipe calls for hot smoked paprika, consider adding a little cayenne to the dish to bring up the heat level without adding any unwanted flavor notes.
You can use your mix of sweet paprika and cumin in exactly the same amount that you would use smoked paprika.
In a pinch: Guajillo powder
Guajillo chilies are among the most popular chiles in Mexico. It is popular for use in moles and in salsas. It is also effective in chili and in soups. This pepper has a sweet fruitiness that some people liken to cranberries. That fruitiness can make it a good stand-in for smoked paprika. When ground, the guajillo chili is versatile enough that you can use it to make harissa and you can combine it with other spices to make a barbecue rub.
The heat level of guajillo chilies can range from very mild to moderately hot. Usually, cooks will add a more pungent chili if more heat is needed so they are not extremely hot. You can usually get away with using the same amount of guajillo to your dish as you would hot smoked paprika.
Generic chili powder is a great option as it is usually a combination of ancho pepper and cumin along with a few other herbs and spices. The cumin gives it a mild smokiness and the ancho gives it some of the redness you would get from smoked paprika.
Gochugaru does not have a smoky flavor, but it does have a bright red color, which makes it a perfect smoked paprika substitute from a visual standpoint. If you absolutely need the smokiness, you can add a few drops of liquid smoke to your dish or toss in a little cumin.