This includes any manifestation of the Dark Force/Profound Darkness. I just ran out of room in the title and went with "Dark Force." This doesn't include other bosses.
Which are your favorites? Which ones disappointed you? Any reasons are acceptable. If you like your enemies better or worse based on their attacks rather than appearance, that's fine! I'm personally basing mine more on the artwork, as that's what's more interesting to me.
My absolute favorite is Dark Force from PSII. The slightly more "realistic"/less comic book-ish battle scenes (except for the unfortunate blue grid backgrounds) of PSII made a lot of the enemies scarier than other PS installments, and Dark Force is the real standout here (excluding Mother Brain). It also retains a lot of the same features and the main attack of Dark Falz from PSI. I like the connection. I like how it looms from above; I picture this Dark Force sort of hanging down from the rafters in the ship. Maybe it's attached itself to the ship, much like the first Dark Force in PSIV.
Runners up include Dark Falz from PSI (its electric blue color and simian arms are really impressive) and the aforementioned PSIV Dark Force. I liked how he was sort of an unfinished mess of nerves attached to the computer system.
My least favorite is the Dark Force from PSIII. I don't mind the departure in style as much as it was such a let down how easy he was to defeat. My second least favorite is the final version of the Profound Darkness in PSIV. It was mostly disappointing because I guess I was expecting something less... well endowed.
Whoa! Sorry for the novel. I'm highly opinionated, it seems.