Most of the most epic or moving moments that I can think of have been mentioned already, so I'll just post a not-so-epic-but-still-pretty-cool moment:
When Luffy got stuffed full of shadows and became Nightmare Luffy and came to save Nami and Usopp. His voice and speech patterns had changed. And he was like 'Ore no na wa..... Monkey D. Ruffi, da ze!' Him using 'da ze' was just classic
Then, he blocked Oarz' punch with one hand, and then he was like "Ruffi wa.... ore hitori, da ze!' That sealed the deal on the coolness.
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وَمَنْ لَمْ يَذُدْ عَنْ حَوْضِهِ بِسِلاحِـهِ
وَمَنْ يَغْتَرِبْ يَحْسَبْ عَدُواً صَدِيقَـهُ
وَمَهْمَا تَكُنْ عِنْدَ امْرِئٍ مَنْ خَلِيقَـةٍ
وَكَاءٍ تَرَى مِنْ صَامِتٍ لَكَ مُعْجِـبٍ
لِسَانُ الفَتَى نِصْفٌ وَنِصْفٌ فُـؤَادُهُ
يُهَـدَّمْ وَمَنْ لا يَظْلِمْ النَّاسَ يُظْلَـمِ
وَمَنْ لَم يُكَـرِّمْ نَفْسَـهُ لَم يُكَـرَّمِ
وَإِنْ خَالَهَا تَخْفَى عَلَى النَّاسِ تُعْلَـمِ
زِيَـادَتُهُ أَو نَقْصُـهُ فِـي التَّكَلُّـمِ
فَلَمْ يَبْـقَ إَلا صُورَةُ اللَّحْمِ وَالـدَّمِ