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!