Hatsusaku is probably the game why people even like Sagaplanets in the first place so try that. Karumaruka is also a fun moege but if you want to play it I recommend doing it after Hatsusaku (because it has minor references) but not a must. Also if you don't mind little older stuff then Natsunagi.
Having played most Sagaplanets' games I can say that Hanasaki is the worst one imo and I don't get why people like it. Nonoka's and Kanna's routes were also the best for me.