I'd recommend getting a logic-level MOSFET, you won't be able to switch 2.6A using
a single BJT as it will need 200mA on the base (far beyond the capability of an Arduino
pin), and a Darlington will need quite a lot of heatsinking at that power level (about 3W).
An n-channel logic level MOSFET with an Rds(on) of 30 milliohms or less will be fine
and won't need any heatsinking, the diode across the solenoid is always required, note.