There are some must have mods for STTNG three I will talk about are:
Installing the "Hallmark" ornaments. One in particular I am going to talk about is the Klingon Bird Of Prey (AKA: KBOP). (Ref: http://www.pinwiki.com/wiki/index.php?title=Star_Trek:_The_Next_Generation#Klingon_Bird_of_Prey_Mod )
Here is the difference in detail....
And once you have the KBOP, you need to install Zitt's modules instead of the original crappy #47 bulb holders. Ref: ( http://pinball-mods.com/blogs/?p=452 )
With ZITT's modules you can trash the ugly old dual #47 holders...
And get a low profile solution that looks really nice..
And of course if you want to run LED's to really make your machine pretty, and if you do and leave everything stock, you will have problems with your GI and especially on STTNG the 'Shields" as they are GI and not feature lamps. The issue is STTNG has the ability to dim the GI and uses that ability to dim the shields as your getting hit in multiball. Unfortunately LEDS + lowering AC voltage do not = pretty dimming.
So you will need a solution like the GI OCD (Ref: https://pinside.com/pinball/forum/topic/gi-ocd-active-smoothing-for-gi-work-in-progress ) to change those AC waveforms into some nice PWM-esque wavefoms to cleanly dim LEDs.