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Gong Hyo Jin has 4 shows that have pretty high ratings. 

1. Jealousy Incarnate (8.3) 

2. It's Okay, That's Love (8.8) 

3. The Master's Sun (8.8) 

4. The Greatest Love (8.2)

Which one of those should I watch? 

Which show has a nice male lead?

It's Ok That's Love was my favorite, followed by Master's Sun. Greatest Love was good too. I haven't seen Jealousy Incarnate yet.
The leads characteristics in Greatest Love and Master's Sun are pretty similar.  While Greatest is rated the lowest, I found it to be more enjoyable than I thought it would be.  The other series I felt like I knew what I was getting in to in regards to the leads chemistry.
Personally I loved all of these and you would not be worse off if you chose to watch them all. Whilst I do love all of them, It's okay that's love and Master's sun are my favourites. In terms of male leads the leads for Greatest Love, Master's Sun and Jealousy Incarnate are all rather endearing if not slightly stubborn and petulant at times. There is a lot of push and pull with those male leads with feelings being questioned and classic hesitancy being found throughout, but this is not the case in It's Okay That's Love, the male lead is just hilarious and witty but still complex and quirky and in my opinion the most interesting to watch on screen. Still I stand by my original statement that you should definitely give them all a go, but in terms of where to start It's Okay That's Love is wonderful, it's funny (great ensemble cast), dramatic, romantic and fluffy with wonderful chemistry but also tugged on my heartstrings on more than one occasion.>_< (side note I happen to love the OST for It's Okay That's Love too so potential added bonus ^_^)
I hope this helps!
Its okay that's love
Masters Sun
Thank You
The Greatest Love is one of my favourite dramas of all time.
It's okay that's love
Master's Sun

I didn't like It's okay that's love...it was weird for me..so I dropped it.
I was almost done with the Greatest Love back then, but I ended up not finishing it.

So in my opinion, Master's Sun was Gong Hyojin's best drama. It was just very cutee and funny.
Same with OliCT, I recommend Master's Sun

I also didn't seem to enjoy It's Okay That's Love
Master's Sun then It's Okay that's Love 
It's Okay, That's Love is my fav. And i liked Jealousy Incarnate a lot too.
I highly recommend The Greatest Love.
I love all of them! They are all in my re-watch list. I suggest you to watch them all in this sequence: 

1) The Greatest Love
2) Master's Sun
3) It's Okay That's Love 
4) Jealousy Incarnate

*Every male lead are nice in their own way. 
Since, GHJ is one of my superrrrrrrrr favorite actress of all time. I would recommend you to watch The Master's Sun first. (maybe because it is the first GHJ-drama I've watch) 
I would recommend the Master's Sun and It's Okay That's Love. However, those are the only ones I've seen so far. Will watch the others later.