One that fits where you need to store it. Then it becomes a question of whether you want a grill, a smoker, or both. BBQ means different things to people in different parts of the country. Third issue is how much do you want to cook at once, a matter of size that can be at odds with where you need to store it.
If you want to do both, the Weber Q series is a high quality product that comes in a range of sizes. It should last a lifetime with occasional replacement of parts, and in the past Weber has been good about parts.
But if it doesn't need to last forever, and needs to be compact for storage, there are a number of box-like sheet metal portable grills with smoker covers and folding legs in the $30-100 price range: Char-Broil, UniFlame are readily available brands.
I can tell you one not to buy, Coleman's "Camp" grill line. While it has a nice big grill surface, and packs up briefcase size, the heat distribution is so poor that most of the surface is not usable. There is a version that has a smaller grill alongside a single stove burner, and it is neither a good stove nor a good grill.