Curiosity gets me to look at ZTX851:
Collector-Emitter Saturation VCE(sat) 10 50 mV IC=0.1A, IB=5mA*
Voltage 50 100 mV IC=1A, IB=50mA*
100 150 mV IC=2A, IB=50mA*
200 250 mV IC=5A, IB=200mA*
Well, quite impressive 250mV at 5A, only there is no way arduino could deliver 200 mA base current.You can't use such transistor even for 1A load, I don't know any micro with 50mA outputs driving capability.
TIP120, for comparison, needs only 3 mA for 3A collector current.
Other things, ztx851 according to "safe operating area" can supply 5A during 1 sec. TIP120 will run 5A forever.
MOSFET's are nice alternative, same time even logic level transistor would require special high capacitance driver IC for most PWM application.
It seems to me, that old TIP120 is ONLY the best friend for microprocessor application, with one nuisance - base resistor