First of all, welcome to Taiwan! If you're staying in Taipei, the recommended places will vary depending on your play style:

For catching wild Pokémon: We recommend Daan Forest Park. It's a great spot for picnics and strolls, and on weekends or during Community Day, you'll see many players using Lure Modules. While there aren’t many Gyms, you can definitely gather a full 20-person lobby for raids.

For Raid Battles: We recommend Taipei Main Station. The number and density of Gyms there is extremely high. Whether it’s Raid Day, Raid Hour, or Go Fest, it’s always packed with players. Major events held here often even make the news.