The Garden features two main seating tiers with a large ring of suites separating the lower level from the balcony. The best 300 level seats (another name for the balcony) are located along each sideline. With ticket prices often 50% less expensive than lower level seats, this is the cheapest way to sit near mid-court.
In addition to unobstructed views of the court, sections along the sideline are much closer to the court than corner and baseline sections. This will help keep you and your guests engaged in the action and make it easier to see in greater detail.
When considering tickets in the balcony, aim for seats near the section entrance at row 3. In addition to being close to the front of the section, you'll be right at the tunnel that leads to the concourse.