In general, casseroles, soup, stews, and meat freeze well. This includes meals like lasagna or any baked pasta - one of the first freezer meals I learned to make was these Cooking Light stuffed shells. Pasta sauce also freezes well, like my favorite bolognese recipe.
A super easy way to make a freezer meal is to put a roast in a zip-top bag (don't forget to label it!), top with your usual seasonings (salt, pepper, garlic, chopped onion or onion powder, etc.), and then freeze. When you want to cook the roast, defrost the contents, dump them into a slow cooker, add your favorite cooking liquid, and cook until tender. For example, I've done this with my Soy Onion Shredded Beef recipe - I put everything but the water in a gallon-size zip-top bag and freeze until I'm ready to cook it. Add a salad or roasted vegetable for a complete meal.
I know it's not a revelation, but uncooked meat freezes very well. You can just toss the meat into the freezer for later, but you'll definitely thank yourself if you turn that ground beef into meatloaf or burgers first. For freezer meal purposes, it's best to do something to the meat speed up your cooking process, whether it's dicing chicken thighs for soup or cutting chicken breast into nuggets. Getting that one step done before freezing the meat can be the difference between "That's too much work so I don't feel like cooking tonight" and "All right, that won't take very long." (Why yes, that is the voice of experience.)
Many frugal cooks like to brown their ground meat and freeze it in one-pound portions so it's ready to go for tacos, etc. but I personally prefer not to do this because I don't like the texture of the defrosted cooked meat unless I have a specific purpose for it, where the texture isn't going to bother me, like in enchiladas. So I do keep one or two pounds of packaged ground beef in the freezer.
As mentioned already, soups and stews freeze well, but if you're freezing the prepped ingredients rather than a cooked soup or stew, I recommend leaving out the liquid (chicken stock, water, etc.) just to save space in the freezer. For example, if you're prepping Crockpot Taco Soup, leave out the beef broth when freezing and add it to the slow cooker on cooking day.
Meals that are mostly dairy don't freeze well because the dairy can separate and change the texture of the meal. But if you have a meal that has dairy added at the end, freeze everything else, and then add the dairy before you serve it.
Casseroles usually freeze great, although I usually throw them together on the day I'm serving them, using whatever I cobble together from the freezer. That's the beauty of freezer cooking - it's infinitely adaptable to how you like to do things, but as long as you use what's in the freezer, you'll save money and usually time too.
What are your favorite meals to freeze?