My opinion:
!) Jouka
The whole song itself sounds amazing. From the opening piano runs as well as the synths, then come the vocals which sound cold and dark yet beautfully sung. Yet obviously, the best part comes from the IIDX RED ending. Too bad though that whenever I play the [h] chart, I make the song sound so crappy
2) Sabaki
Opening guitars are there. Though when you look at it, when you compare the GFDM V2 opener with the one from IIDX GOLD, there may not be much difference after all. I mean, add the vocals and you would feel A/I's voice to be sweet and euphoric. Take away the vocals, then you can appreciate the guitar plucking more. Nothing to gain and nothing to lose by removing or adding the opening vocals.
3) Chikai
At first, I thought it sounded crappy. My first worry was that her voice was degenerating (much like my initial fear years ago that TERRA/Junko Terashima was having the same thing as I felt that her singing voice was going downhill after DoLL). Though after playing it more, it kinda grew on me. As usual, the best part comes after the vocals as these are all piano runs then this goes on as it reaches the Custom Drive theme song part.
So personally:
JOUKA>SABAKI>CHIKAI
Needless to say that all of them are neat songs to play with
