2.125 is a width that is common on cruisers, and generally they are heavy and slow tires. There could be better tires made in this size but I am not familiar with them. Nicer tires are made in 26x1.9, 2.0 and 2.1 and 2.2 and loads more sizes.
Any 26" x (width indicated by a decimal number) tire will fit, so you can choose lighter, more robust, more tread, less tread, flat protection, etc. Try to keep the width at least as much as the width of the rim, and at most so the tire is not rubbing on the frame and fork, but otherwise you can put whatever tire on you like.