w5w simple 5mm

minimalist version to support 5mm leds and with smallest possible footprint. actually dual or tri 3mm would be brightest and most economical version imho .

edit: added also dual version. connect smd resistor inbetween leds and glue it all with abs juice.

edit2: smd added improved dual version. it is slightly bolder than standard bulb so it may not fit some sockets (most 'springy' ones have enough tolerance for tight fit though) it also got room for subminiature 0.125W through-hole resistor .
you still need to sand LED's a bit or use 3mm ones.

HINT: for white light you can get "12V" leds which look like ordinary 5mm led, but do contain three structures connected in series. they are three times brighter ofcourse.