The Guandu temple was built in 1661, but it's not an ancient relic. It's a huge temple, about a city block square, a couple of stories high. Everywhere you look, there's amazing art. The roofs are intricately decorated, and the back of the building has a huge wall of detailed stone carvings. There is also a passageway with a variety of wood and metal carvings.
The grounds around the temple are also gorgeous; there's a small waterfall, and there are staircases (also with delicate carvings) that lead up to an area above the temple. The view of the surrounding area from there and from the top of the temple is spectacular. Expect to spend at least a couple of hours to see the amazing beauty of the place.
To get there, take the metro Red line to Tamsui and get off at the Guandu stop. There are buses that go to the temple at half-hour intervals. It's about a one-kilometer walk from the metro station to the temple, so, in terms of time, it's a toss-up. There's an open-air food court about a block from the temple.
Yes, there are other temples in Taipei (Longshan Temple and the Confucius Temple), but, as far as I'm concerned, they pale in comparison to Guandu.
This review is the subjective opinion of a TripAdvisor member and not of TripAdvisor LLC.