There is a first for everything, but for me the most "influential" is the one that makes that trope stand out the most and not necessarily have to be the best. Those I consider classics, an equivalent of modern-day Shakespeares' plays.
Hana yori dango franchise rich boy/poor girl-parents that are traditional when it comes to marriage The manga came out in 1992 and includes the the following tropes
Kabedon Amnesia Rich boy poor girl Spunky tough FL (or Weed Lead) Angry on the outside, naive and squishy on the inside ML Evil parent Stranded on an island Cohabitation Bullying School Romance Momento (had to think which trope the necklace falls under) Love triagle Bromance
Meter Garden is influential, I think it's probably the most influential out of all the adaptations. I say that in terms how incredibly popular it was in East Asia and South East Asia at the time. It popularised the story even more.
For Wuxia, it'd always be Jin Yong's. His most popular works is Legend of Condor Heroes I think? Considering how many times it was adapted. You can't really talk about how a specific version influenced dramas though because it's his novels that popularised and influenced the genre. Non Jin Yong well known wuxia dramas are by other well known wuxia writers from same time period.
Xianxia would be Chinese Paladin? which, again isn't the drama influencing other dramas but the original game (1995) creating the genre, and it was adopted and further popularised by web novels, which then got adapted into dramas. The drama is notable for amongst the first (2005) to be aiming for the younger audience. Previously most cdramas were more general audience/family kind of tv.