It depends on what genres of stories you like the most. Here's some pretty popular manga that are good to start with:
- Berserk
- Monster
- Hajime no Ippo
- Vagabond
- Yotsuba&!
- Azumanga Daioh
- Dungeon Meshi
Whenever you add manga to your ebook reader, I recommend first using Kindle Comic Converter to downscale the pages to your device's resolution to save space and reduce loading times.