I read the small synopsis of all in wikipedia, but I don’t want spoilers.
The summaries didn’t say much to me. I feel more incline to reply 1988, since I think that’s the one with PGB, but I know nothing about the others. Only that they are all very famous.

I would say Reply 1997 – Reply 1988 – Reply 1994

1988 is my favourite but it’s incredibly long per episode, so I wouldn’t suggest it to somebody who hasn’t seen any reply series yet. I would suggest to see 1997 first.

1997 is my favorite and I watch it again and again. I don’t rewatch the other two. 1994 was my least favorite. 1988 was great until the end, but the last part was not so great in my opinion.

For me 1997 is leaps and bounds better than the other 2. It was fast paced (~45 min eps against ~1.5hr for the other seasons) and had the right concoction of nostalgia and romance.
1988 was also well done, specially in the family+friendship department. The romance, well I preferred the side couple rather than the main ‘who is the husband’ mystery.
1994 was nice, the gags were good, but the episodes could drag. The friendship was great to watch! The romance, well I’ll leave you to make your own judgement about it.

So chronologically 97 -> 94 – > 88 (as 13infamyass mentioned, there are cameos from the previous show in each subsequent season)
My preference 97 – >> 88 – > 94

I throughly enjoyed each and every one of them, but 97 is in my top 10 all time list

I agree with 97 first (its the best n the husband mystery wasnt meant to toy with the viewer). However you like the best fr last right?
88 is great but really looong episodes so may feel it a bit tedious..
94 havent watched it yet not sure if its worth the hours..

I’ve only watched 97 and it was great. I’d rec reading the recaps too of you can swing it. They give a little bit more insight.

I haven’t seen Reply 1994.

If you want fun, crazy nostalgia, sizzling romance: Reply 1997.

If you want deeper look at family and friends relationships, with a calmer vibe: Reply 1988. (It’s my favourite – Beware longer episodes though)

I liked 1988 the most. It felt really homely and focussed a lot on the families instead of just having the spotlight on the main couple. And it also gave me the most conflicting ‘second male lead’ syndrome ever! I just couldn’t decide who I wanted to win because I liked both the guys.

1997 is my second favourite. I loved it like anyone else but at times I felt she didn’t really deserve him!

1994 – completed it on my second try. Go Ara’s ‘pursed lips’ became too much for me at one point but I was determined to see it all!