The standard M&P 15-22 comes with very reliable 25rd magazines. They have only one down side, the open construction to facilitate loading. They are also out currently of stock with most vendors. Two alternatives both raise capacity and improve sealing.
Plinker Tactical makes a 35rd magazine in several colors to match the M&P15-22 finish options.
Internally, it’s a double-stack design with a strong spring. To make loading more comfortable, a special device is available from P.T. that fits in the back of the magazine over the hold-open latch.
The scallops on the back are finger-grooves for de-tensioning the spring during loading. I found this drum design to be very reliable and dirt-resistant. These don’t fit standard AR15 mag pouches, but BDM offers Molle drum pouches that work well.
Promag also makes a 32rd magazine similar to the M&P design but I do not have one on hand for the comparison.