Well, for me, I really do enjoy listening to the heroines themes/leitmotifs despite the fact they do overshadow by the better tracks in their respective OSTs. I’ll list the ones that I like in every series. (In no particular order)
Kanon:
Shoujo no Ori - While it is certainly repetitive, I personally like the mysterious, eerie vibes and atmosphere that the music was giving to me every time I listen to this. Also, I find it really different and isolated from the other character themes in Kanon, and it really does certainly fit with Mai’s character.
AIR:
Natsukage - I don’t think I have to explain why… 
Niji - I’ll give Minagi’s theme an honorable mention. I can’t completely explain why I like this but I just find this music kind of catchy for me.
CLANNAD:
Like the Wind - I really like the arrangement and the upbeat melody of it and very much like Kyou’s personality. And for some reasons, it gives me energy every time I listen to it. Plus, it has a vocal version which is ‘Over’ that I really like.
Etude pour les petites supercordes - I really like how unique and catchy this track was. I also like the use of string instruments here. And like what was said earlier, it is different from the other character themes of Clannad. and it certainly reflects Kotomi’s eccentric personality.
Tea in the Reference room - I really love this one. It kinda reminds of a certain video game soundtrack that has a similar style with this one. It really makes me feel calm and just want to relax.
Little Busters!:
Approaching Light - Mio’s theme is also one of those tracks that is different and separates from the other character themes in the series. It’s really simplistic yet so beautiful, and has a sense of isolation, very much like Mio.
The Grief of a troubled girl - One of the most memorable track for me. Just hearing it will definitely remind you of a certain troublemaker girl. It’s so catchy and playful, and the random arrangements just really reflects Haruka’s personality overall.
Will & Wish - What I like about this track is that it juxtaposes Haruka’s character theme very well, much like how Kanata juxtaposes Haruka and vice versa. While Haruka’s theme shows the quirky, randomness and her playful nature, Will & Wish shows the serious nature and the sadness that Kanata has within her. It’s a good track for me.
Cat, Glass and the round Moon - Surprisingly, I like Sasami’s theme as well. It’s actually kinda sounds similar with Rin’s theme (or maybe that’s just me) but a bit mellow, I think. I can’t completely explain why I like this but it’s just so catchy for me. Plus the Glassware, which is the piano version of this is also good.
Rewrite:
Anthurium - Since I keep hearing this most of the time in the VN, I ended up liking it, plus I find it really catchy personally. It has that feeling of mysterious, yet playful vibe in it and it feels ‘at home’ and welcoming every time I hear it. And I think it fits Akane’s character.
Nirinsō - I like the atmosphere of this music. It may seem that it doesn’t fit Kotori’s happy and goof ball look and personality, but every time you hear it, you will really think that there’s more than to that smile that she shows in front of you. And you’ll get it once you get to know her more.
Harmonia:
Cantus - I have not much to say here, aside for the fact that it’s simple, cute and very orange.
Summer Pockets:
I don’t have much to say here since I haven’t read it yet, but I listened to them for awhile and I would say I’ll go for Golden Hours and Adventure for Black. I like the energy of their music especially Golden Hours, it feels sort of like a JRPG soundtrack, it’s really catchy.
