You sure have raised the bar with that opening post... good thing it's the weekend now so I can actually put some thought into this.
One thing first: Until like, 2013-ish I didn't care for most groups outside of MM and Ayaya as well as Maeda Yuki. That's when I just got interested in S/mileage, that's when Juice=Juice was formed. It also means that I missed lots of debuts. Maybe these songs sounded great when they were released but... I realised that a lot of them really don't have that much of a punch. I actually prefer lots of recent debuts.
1. W - "Koi no Vacance"
One of the few exceptions. I love everything about this song, the MV, even the original - and listen to it on a regular basis. Great debut!
3. Juice=Juice - "Romance no Tochuu"
Can't go wrong with the typical H!P disco-funk-jazz sound. Only negative point is that it becomes a bit repetitive.
3. Aya Matsuura - "Dokki Doki! LOVE Mail"
Another older one, but Ayaya's charme here is amazing. It's cute, without being the sugary-idol-cute.
4. Angerme - "Taiki Bansei"
See Juice. This is pretty much the type of song you need to hype a group.
5. Kobushi Factory - "Dosukoi! Kenkyo ni Daitan"
...or something like this. Back when Kobushi was still a fist in your face. Powerful.
6. S/mileage - "Yumemiru Fifteen"
A classic. Probably wouldn't have thought of it like that back in the day but nowadays it's actually one of my favourite S/mileage songs. The dance is catchy, too.
7. Country Girls - "Itooshikutte Gomen ne
Like a hand in a glove. The cheeky tongue-in-mouth mood of this one was great for CG's debut. Oh, what could have been...
8. Tsubaki Factory - "Hatsukoi Sunrise"
Tsubaki's indies? Shit. This one? Bam. It was the first Tsubaki song I liked.
9. Minimoni - "Minimoni Jankenpyon!"
It's a bit like Country Girls: You get what you expect. Fits the mood of the group really well.
10. Tanpopo - "Last Kiss"
Another classic. There's quite a lot of emotion in this, and few groups other than Tanpopo can deliver it.
11. Heike Michiyo - "GET"
Might not be the greatest song ever, but it... got me. Heh. Love the desperate "Suki da tte".
12. Morning Musume - "Morning Coffee"
Sometimes I wonder if I just got used to it. But then again the harmonies are also quite nice.
13. Pucchi Moni - "Chokotto Love"
Can't be compared to Baby Koi ni Knock out, but still a nice, catchy song.
14. C-ute - "Sakura Chirari"
It's cute, what it says on the tin. Fits the little girls.
15. Berryz Koubou - "Anata Nashide wa Ikite Yukenai"
I think back in the day it was still a bit too big for Berryz - it was cool, but nobody really wanted try-hard kids acting cool. That alone makes it really interesting, though.
16. V-u-den - "Koi no Nukegara"
Well, V-u-den. Not their best song, but it's not a bad song.
17. Yuki Maeda - "Naki Usagi"
As mentioned before I was a sucker for Yuki's enka.
18. Miki Fujimoto - "Aenai Nagai Nichiyoubi"
To be fair Miki's career didn't really start until Romantic Ukare Mode. It's decent though.
19. Koharu Kusumi - "Koi☆Kana"
Why would Koharu of all people rank relatively high? Because the song fits her character and it's not the worst song on this list.
20. Country Musume - "Hajimete no Happy Birthday!"
One of Country's most annoying songs, sorry.
21. Yuuko Nakazawa - "Karasu no Nyoubou"
The slight enka note gives it something interesting which fits Yuuko's mature character, but it's not that great.
22. Buono! - "Honto no Jibun"
To be fair I really just didn't care for Berryz or °C-ute back then. There's pretty much only two songs by Buono that I really like: Rottara Rottara and Hatsukoi Cider.
23. Taiyou to Ciscomoon - "Tsuki to Taiyou"
I remember that I said T&C had a great debut. But yeah, debunked - the first single was quite a lackluster. The next 4 however are sweet.
24. Maki Goto - "Ai no Bakayarou"
Who wrote those "I cry, cri-hy, cri-hy" lines? Damn.
25. Coconuts Musume - "Halation Summer"
Yeah, everything about CM was messed up, we know that. Their debut was bad, but it wasn't the worst in this list.
26. Erina Mano - "Otome no Inori"
A solid idol song, but Mano had very few catchy songs. But songs like these are not what I'm here for.
27. Melon Kinenbi - "Amai Anata no Aji"
28. Natsumi Abe - "22sai no Watashi"
I'm still surprised they'd give their ace such a sad debut. Maybe they wanted to make her look mature and cool. Didn't work.