Epic games Fortnite is a very popular online multiplayer FPS with over 15 million players the community is really huge and is growing ever since release.
The game is really well made and optimized for PC's and therefore a low budget PC can also handle it. The game offers various battle modes from Battle Royale to solo. It also has a great developer support and the developers keep the game fresh and interesting by adding various characters and and add-ons as the time passes.
Link:https://www.epicgames.com/fortnite/en-US/download
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#3. PUBG PC LITE
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#2. VALORANT
Valorant developed by riot games is also a fantastic Multiplayer online FPS shooter for PC , having a lot of stuff similar to CSGO but also different in many aspects. The game has a unique animated character design who posses abilities and various game modes such as competitive, unrated and especially unique Replication(play to find out.).With over 10 million players a month the game obviously has a large player base which is growing rapidly.
Link:https://playvalorant.com/en-us/
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#1. CSGO
Counter Strike: Global Offensive is at the epitome of FPS Multiplayer gaming with more than 20 million monthly active players. It is not a surprise that it is developed by valve which usually makes the best selling titles. It is free to play so you can just create an account on steam, download it and start playing. Though a major change in the form of Prime Accounts being restricted to only paid players has been made which may upset a lot of players but still it is the go to multiplayer game.
Link:https://store.steampowered.com/app/730/CounterStrike_Global_Offensive/
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If You have any doubts or suggestions, please let me know in the comments.