I put WA DA DA at number one because it's just a giant load of fun. It's both cute and powerful, and even though I didn't watch GP999 and knew next to nothing about the members, a few of them really caught my eye with this song alone and made me want to know more.
FEARLESS is #2. A simple yet catchy song, a fitting concept. Plus Kazuha as the ace up LE SSERAFIM's sleeve.
SHUT DOWN is #3. Unlike GP999, I followed My Teenage Girl so I was looking forward to the group's debut. The song is nice, relentlessly energetic.
Then, ATHLETIC GIRL was surprisingly good. Give it to a group with a longer life expectancy and change the lyrics and concept for something less workout oriented and you have a hit.
BOP BOP was really enjoyable too.