The best bird to go with a cockatiel is another cockatiel. If you can't get another tiel, then my next suggestion would be a bourke parakeet. They're like the cockatiels of the parakeet world.
When looking for a companion for a tiel, a good rule of thumb is never get another with a bigger beak than a cockatiel. Also, you need to look at personality. Budgies are feisty and hyper. Parrotlets are fearless and pushy. Lovebirds are crazy and can be aggressive. So, it's best just to stick with it's own species