That depends largely on where you live and what species are present during a given time. We have found that having a diversity of type and number of feeders works best. Larger birds such as Blue Jays are very competitive and will keep smaller birds away, if you don't provide enough feeders and space between them. Ideally one large hopper type feeder with access from multiple sides allows several birds to feed concurrently. Place the large feeder on a pole, and use a baffle to keep the squirrels at bay.
Baffles are available at most hardware stores.
Two or more others of different types placed in adjacent trees or atop poles serve to create a sort of sanctuary where a large number of different birds can gather to feed in safety. Niger seed is expensive compared to sunflower seed. But again, buying in bulk will save money while offering the reward of seeing 15-20 brightly colored Goldfinches vying for an open feeder perch. Ours is hung in a tree with some small limbs nearby that serve as a waiting area while all the perches are full. Another type feeder that is popular with woodpeckers, Titmice, Nuthatches, and many others is a wire mesh tube filled with unsalted peanuts.