In a recent study, the 6 line ranked worst for Wi-Fi and cellular performance, along with the J, Z, 4, and 5. Having issues getting connected on the NYC subway? New research suggests you probably ride the 6, J, Z, 4, or 5 lines. Independent network testing firm Global Wireless Solutions on Wednesday released the results of a new subway data test, indicating that the Q line has the best overall service on mobile phones.