1 day in Delphi or 1 day in Crete..?

It really doesn't make much sense to visit Crete for a day. The ferry from Santorini gets in a around 7 pm in Heraklion so that's your one night. The only reason you might want to do that is if Knossos is number one on your bucket list.

It makes way more sense to add that day to Meteora and make a three day Delphi Meteora trip. They are wonderful completely different sites. Having said that connections between the two sites is pretty difficult so you would need to rent a car.
Maybe this will help you decide
Olympia Delphi Meteora https://www.flickr.com/photos/stanbr54/sets/72157645469717811/

It looks like you are spending, at most, a week in Greece and are trying to go to 4 different locations? For just one week, 3 places is all you should really be trying to do

I suggest you add the 'extra' day to one of your locations, and spend some time 'being' there , and not so much time on the move.

PS - If you do choose a 4th, I vote for Delphi.

Going to Crete for one night doesn't make sense at all. You'd have virtually no time to spend on the island.

How are you getting to/from Meteora? If driving, it would be easy to add a stop in Delphi. If taking transit, then spend the extra time on Santorini or in Athens. Don't waste even more time sitting on buses.

Also know that the town of Delphi is nothing to write home about. The historic site and museum are excellent and worth a visit if you have time; just don't plan on spending time in town other than eating and/or sleeping.

Hey, thanks everyone for your input! In retrospect, I definitely agree, I am trying to cram too much into the trip... I am learning a lot through all of these responses, and it's definitely helping me plan the trip out better. Thanks again!

Jason