Ask me last month, and I would have said Stranger. Now, it is Lost. You mentioned you like romance. If you like it clear-cut, this might not be for you - the leads relationship is one that cannot be easily defined. The romance is a very subtle undertone, as much of their relationship is a healing friendship, but wow, do they have amazing chemistry.

Battle of Changsha. I've watched it about three times now. It's got almost everything that I like: history, romance, family, action. As long as Arthdal Chronicles has a good definitive ending in the future it will probably become my number one since I do love historical fantasy a lot.

The Good Detective or Mad Dog. Idk it’s just something about the badass characters in these two that makes me want to watch it over and over again. Vincenzo was the same but I can’t get the same novelty in it by rewatching it. The first watch was golden. 

i would say 1 litre of tears

I cried a lot, it's really touching. It's probably the serie I've been most emotionally invested in

An impossible task, but  of the 4 Dramas (out of 425 finished) I've watched 4 times each,  this one has yet to grow stale

Okitegami Kyouko no Bibouroku

So hard to choose only one. I have several favourites. But if it has to be, than I recommend you:

Majo no Jouken

Is this a drama series?

 emaureeng:
Cloudy. Rain beginning early in the morning. Wind becoming southeast 40 km/h gusting to 60 near noon. High plus 5.
 NightPeriods of rain. Windy. Low plus 2.

Can't say one favourite of mine, there are so many. But just for romance, this is not bad at all in my view, Love Of Thousand Years

It's also hands down Nirvana in Fire for me, but if you need more romance that I will say Rise of the the Phoenixes (romance is prominent there IMO but there is still a lot beside it). Unfortunately all my other favorites don't really focus on romantic storylines or hardly  have any.

But as for romantic dramas I somehow keep coming  back to Bloody Romance (the most touching albeit underplayed  romance for me) and to The King Loves ( sweet, heartfelt and rather light) although I would probably not ranked  them that high overall. 

Handsome Siblings (wuxia, my favorite)

But if you want romance, then Romance of Tiger and Rose