PDI are currently the best in AEG tighbores, followed closely by Prometheus, which I'd suggest you to get. Prommy' are cheaper than PDI yet high quality, and offer a good bang for your buck.
Madbull tightbores have (or had? Ive heard it changed it recent versions, but it has not been confirmed) an inner coating which will eventually peele off, destroying your accuracy and possibly damaging your hopup.
As the other said, hop-up is (together with the tightbore) the most important thing to get good accuracy. Prometheus hopups are good, but I've previously been suggested a Guarder metal high performanced hopup chamber with a Guarder hopup rubber (soft for low velocities, harder for higher velocities) for its cheaper price yet good performance.
But don't take my word on the hopup, I may have misundestood but I think that was it... Either way, it still gives you a direction in which you can search.