Get Quake 2
The hit game from 1997. Redefined multiplayer and mod support. This game helped launch accelerated graphics cards into the main stream. With a vibrant modding community that continues to this day. The Legacy of Quake II stands.
You don’t need to purchase Quake 2 to play it online. You just need the starter and mods loaded to play. This is the preferred method to get started.
You can, of course, always purchase Quake 2.
Play Online
If you use the Quake 2 Starter, you can play online, but you will be missing some textures, but the game is still playable.