This next game will give you some nostalgia if you ever played Counter-Strike, CS: Source, or CS Go.
Critical Ops is another excellent multiplayer first-person shooter game for iPhone that I absolutely love. It's an extremely fast-paced game that will test your skills, reflexes, and accuracy just like Counter-Strike back in the day.
C-Ops is a military shooter with terrorists and counter-terrorists, bombs, grenades, guns, and much more. You can fight and join battles with your friends, join servers and play on your own, and climb the leaderboards. We like the fact that it's completely free, but you can buy combo packs. Don't worry though, it is not and never will be pay to win. The in-app purchases are only customizations and styling, and won't help anyone have an advantage or get a better aim.
If you want to play a fast, fun, skill-based mobile shooter this one is for you.