Hallo Abhishek,

I would take a BC337 or BC817 (smd version). Typical collector saturation voltage is 0.025V at Ic = 10mA, at 25°C and with Ic/Ib = 10. This gives an error of about 2%. If you take a handful of these extra cheap transistors, you might find a type with even lower saturation voltage. Minimum limit is given in datasheet as about 6mV!

But if constancy of intensity is so important for you, why not having an intensity regulation with the help of a photodiode? I know from lamps, which loose intensity when becoming older! So, only stabilizing supply voltage will not guarantee constant intensity.

Kai