In my opinion the seven are:
1 - Mickey
2 - Aqua
3 - Riku
4 - Ventus
5 - Sora
6 - Lea
7 - Terra/Lingering Sentiment
Mickey - is an obv choice, powerful in the light, Keyblade Master, and one of the oldest Keyblade Wielders in the series
Aqua - also obv she's a Keyblade Master, one of the best mages in the series, nd beat both her fellow classmates who were both possesed and powered up by dark entities
Riku - the newest Keyblade Master, apparently built up a tolerance to darkness, nd beat Young Xehanort, and Sora fighting with Ven's style nd armor so technically he beat Sora and Ven
Ven - has a heart of pure light even if he got it in an unorthodox way
Sora - is tha main protagonist pretty much beat just about everyone there is to beat nd is the "key that connects everything"
Lea - is in because he has a keyblade and he's learning how to use it nd just because he went back to the radient garden doesnt mean he's not training there, who knows Sora might be helping him, after all Sora left to train too, besides if Sora gets beaten someones gotta be there to bring him back
Terra - is iffy because Master Xehanort did say they have one of the lights ie: Terra since he fused wit him
but the Lingering Sentiment is still out there and still training/fighting to get its body back which if it had more time after the X-Blade was destroyed probably would have gotten his body back if it knew how, Terra beat Master Xehanort (even though tht was his plan) so he probably wont remain captive for too long, The Lingering Sentiment beat Terra-Xehanort which was a more powerful version of Master Xehanort
Imma rule out Kairi out because shes one of the POH's i doubt she's in she is one of the "lights that restored the world" and one of the "7 lights" to protect the "lights tht restored the world"
Theres no way Erauqs is one of the lights he died in BBS if u watch tha cutscene you'll see his body fade away into tha lights but u wont see his heart float up like Sora's did in KH cuz Terra absorbed it.