1) He carries positive attitude no matter how dire the situation is and his presence lighten the mood and further spreads the positivity to his surroundings. His attitiude literally eases the atmosphere. It is always likeable whenever you have positivity surrounding you.
2) He is funny and even most of his fighting techniques are goofy but effective. Never got bored with his gags.
3) He never doubts himself (except for once) and at the same time he has attitude of knowing what he needs to do to overcome the insurmountable wall in front of him instead of crying in a corner and also knows that he can't do everything by himself. He is confident and arrogant but he knows gratitude.
4) He won't discriminate based on race/sex/religion/status etc. He recognized momo and berated Momo to step up instead of living in the shadow.
5) He is not badass but his badass moments are impactful. Examples: putting hat on Nami, smiling before his death, punching celestal dragon, explaining about pirate flag, destroying Nami room in Arlong park giving freedom to Nami, ringing golden bell, motivating his crew mates when he himself was in crisis against Katakuri, intentionally taking virus in Udon etc. Some of his epic one liners have too much impact and embodies the theme of One piece.
6) He wants to become Pirate king not for some kind of lofty goals. For him simple things like catching beetle bug is equal to becoming pirate king. He is the type of guy who enjoys journey than destination.
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@ImmaIvanoM said, his decisions can still surprise you.....
sorry for bad English, I will put more when things come to my mind.