best way to script doom videos?

I want to make small videos in a game that uses the doom engine. Half of the videos will be scripted sequences, while half of it will be somebody (probably myself) playing through levels. For the "real scenes" im probably going to use a demo format, and stuff like turning off the hud. I don't know what scripting engine to use for the ... scripted stuff. Here is the question. Should i use DED (Doomsday), ACS (Skulltag) or Fragglescript (Legacy)?

If you're intending to use demos, Skulltag or other ZDoom-based ports are most definitely not the way to go, since demos are very likely to break between versions. Can't say I'm familiar with the capabilities of the other two, but if you go the Fragglescript route you may want to consider making it with ReMooD, a Legacy reboot that's actively being developed/maintained (although I'm not sure how it fares when it comes to demos, either -- someone else will know, though).

ZDaemon's demos are quite stable, as newer versions can read old demos. Sadly, ZDaemon doesn't support every ACS fancy stuff though. Maybe a combination of ports could help.
Here's an example of a Doom fragmovie using ZDaemon demos and ZDoom cameras.

how do i use zdoom cameras? Does it require doom builder 2 or is it something i can spawn ingame? also, how would i put camera's in demos. You guys didn't say which port is the best for scripted sequences.

(G)ZDoom is by far the best for scripted sequences.

Cameras are controlled by scripts, so read up on ACS. The choice of editor is mostly irrelevant as long as they support one of the two available scripted map formats (Hexen format or UDMF). DB2 will do the job just fine, though.

I don't really see why you should worry too much about demo formats if you're going to make videos. Unless I guess your plan is "I demo a run of a level, wait 10 years, and then I record the demo playback as a video".

Well in that case, just keep a copy of the GZDoom you used to create the demo.