A lot of their songs lol. But to name a few:
- Sprinter is probably the song with the most overlapping vocals, especially overlapping melody lines
- Aria - The 4-man live with Maya is still the best version of the song because there are 4 vocalists
- Eden - a very vocally demanding that absolutely drains your stamina if you're alternating between two lead vocalists (like seriously try singing Eden solo, it's a much harder song that you think)
- Hikari Furu with the section from "hakanasugite" onwards
- Signal - overlapping melody lines with Hikaru's Engrish and Wakana's siren wails
Keiko (and occasionally Hikaru) on lower harmony IS the "Kalafina sound" and is like 99% of the reason why the melody lines sound so much better than if they were sung solo all the time. Add to the fact that Kalafina's songs are much more vocally demanding than your average Jpop songs and you need 3 people on stage to last through a 22-song set list, which is also on average longer than most concerts that typically have 15-18 songs and usually have audiences singing along so artists do less work (insert Justin Timberlake memes here). You need a LOT of vocal stamina for Kajiura songs. Remember that one FJ live which was basically Kaori and her 3 backup singers? Yea she didn't sound so good by the end of that, and Kaori is already someone with incredible vocal stamina.
The intention when they first did lives according to Kajiura is to best recreate the studio sound on stage, which they quickly found impossible with just 3 vocalists (even with FJ and its 4 vocalists it's just not possible to do so live without a bigger ensemble, which Kajiura has learnt and thankfully incorporated more of in recent years).