best Wi-Fi adapter for iMac

A shared house introduces one or more people streaming video, music, or downloading large games, and that will smack down your perceived Wi-Fi throughput in a hurry. There can be factors in the home construction, network configuration, or Wi-Fi interference (both internal and external) that can limit your bandwidth. Your ISP Internet plan may or may not be a limiting factor for the combined household Internet activity, as can the capability of the ISP (or your own) edge device (e.g. cable modem), router, switch, and Ethernet cabling (e.g. Cat5E or newer). If you are on cable, anyone between you and your ISP head end may be doing excessive network traffic or running a high-traffic web server from their home and that will impact you. Have the ISP run a test to your home to ensure there is no ISP correctable issues involved.

I would pay attention to the first paragraph, upgrading as necessary, before thinking about a third-party Wi-Fi adapter that may or may not be compatible with Sequoia, or that may require a kernel driver, something you want to avoid at all costs. Unless your Mac hardware is really old, the built-in Broadcom Wi-Fi chip should be able to handle what your network infrastructure throws at it.

What's wrong with the built-in Wi-Fi?

Just to add, I have 1GB Ethernet service but on Wi-Fi I get about 500+Mbps d/l and 250+bps u/l