I would say watch reply 1988.
Reply 1997, and reply 1994 are similar. you can compare them.. between these two i enjoyed reply 1994 more. but both weren't memorable for me and lacked the most important element in this kind of series, that is the strong desire to go back to those wonderful days of youth.
Reply 1988 is something different. it is not just my favorite drama among the reply series, but also my favorite drama of all time.
Strong characters that you will get emotionally attached to, strong nostalgic feelings, perfect casting and everything about it was perfect. i cannot say that you will enjoy it like i did, because everyone has different taste. but i can say that it is not comparable with the other two (definitely).
reply 1997 was like any other RomCom drama. fun while you're watching, nothing very special about it. pace was fine, did not drag.
reply 1994 was good as well, but it did drag a little. PS. The reason why i liked this more than reply 1997 is because the characters are more likable.
reply 1988 takes you to the past. you will feel like the characters are your family. and when you're done with the drama, this is when you will feel heartbroken because you will miss it.