What is the Doom monster who would benefit the most from the Spectre transparency effect?

[DemonCake]

Cyber devil. Jesus Christ.

[Async Unicorn]

Lost Souls, they're already a pain in the ass, but with transparency they would be your №1 Most Hated Doom Enemy.

[Jason Maguire]

This wad uses only transparent monsters, and I think has all monsters besides PE, cyberdemons and spider demons: 

If you want to see how the different transparent monster types actually play, you could check this wad out (though it's mostly an outdoor, wide open areas sort of map which may not be entirely indicative). It's possible other wads have done something similar before but this is the only one I know of.