It really depends on your personal interest. You can use some of these websites to discover new channels.

Telegram Channels List: Discover interesting channels for your Telegram
Allows you to browse by category and find top channels in that particular category

English-speaking Telegram-channels. Statistics, analytics, TOP chart. Telegram Analytics.
Also allows you to search by category and top channels. There are also some nifty stats for those who are interested.

Personally I follow:

@telegram - for latest updates on Telegram.

@programmerjokes - for the programmers out there

@braingrub - for bite-sized bits of facts

@washingtonpost - for the latest news

@Curiosity_Tea - for interesting and weird facts

Good luck! :)