Salt - you might not think it is a big deal but, you are missing a bit of the boat if you are using standard table salt in your cooking. Go and get a decent sea or kosher salt. I like flakes but, I do not do a lot of baking. If you are a baker, then you will need a finer grain rather than rougher cut version.
Pepper - Buy whole pepper and a pepper grinder. The stuff that is pre-ground has actually lost much of the flavor profile. Grind what you need, when you need it, to get the most bang in your cooking.
Finishing Salt - Much different from your standard salt we shared above, this favored version comes in many varieties and can add a depth of flavor that you might not otherwise find. Try one out and see how you like it - the Smoked Paprika version I use a lot on chicken, turkey burgers, and on couscous.
Cavender's Greek Seasoning - If I could only use one product to flavor my food, this would be it. I use it on everything from salads, to cottage cheese, to steaks, and beyond. Made in Arkansas, you can get it in a Wal-Mart near you or order directly from the family online.
Zatarain's Creole Seasoning - For times when you want a bit more kick than what Cavender's will provide, this is a great stand in for rice, shrimp, chicken, and chili.
Old Bay Seasoning - If you cook fish, then you need this in the cabinet. Other stuff can come close but, there is no replacing what this can do to add flavor to that Tilapia fillet. I recently discovered their garlic and herb version (which I happen to like a lot) - there are a couple of other iterations but, the standard will never do you wrong.
Garlic & Herb Dry Rub - We eat a lot of chicken and pork, this is excellent to add flavor prior to grilling. Not as messy as a wet sauce, this also helps to lock in flavor/juices during cooking.