Bohemia Interactive's Arma series remain almost uncontested as the games to own if the idea of realistic infantry combat tickles your fancy. The only barriers to entry have really been your willingness to deal with a game that prioritises realism over entertainment, the need for a relatively powerful PC, and the not-insignificant cost of the game itself. The Humble Arma 2018 Bundle fixes a third of those, offering everything in the series to date (minus the latest wave of DLC) for $20.
So, here's what you get at the various tiers:
$1/£0.75 gets you the original Arma: Cold War Assault (formerly known as Operation Flashpoint from way back in 2001, as you can see in the rather aged trailer below), the Gold edition of the first Arma, plus turn-based spinoff Arma Tactics. Of these three, only Cold War Assault is really of any note, as the original Arma was an awkward half-step into a new era for the series. While I've not played it myself, I also gather that Arma Tactics is a bit pants, so probably not worth the price of entry.
At $15, you get Arma 3 - latest in the series - plus its first round of DLC, including the Helicopters pack (introducing an optional-but-fun advanced flight model for rotary-wing aircraft), the Marksman pack (for the snipey gits out there) and the... Karts pack, because sometimes an April Fools joke turns into something real. Arma 2 was where the series really started to hit its stride, even if it was only an iterative improvement on the original, mechanically speaking, but Arma 3 feels like a major leap forwards, both mechanically and thematically - while previous games had modern or historical settings, Arma 3 mostly uses near-future tech.
Thanks to there being so few takers on the lower tier, the Beat The Average package is $15/£12.05 at the time of writing, which gets you Arma 2 and all of its DLC, including the Czech Republic army pack. But really, what you're here for is the full $20/£15 package, giving you Arma 3's major Apex expansion on top of all that. Given that Apex normally costs £25 by itself, it's not exactly surprising why everyone is just opting for the full package here.
As mentioned, this isn't the entirety of Arma 3 to date - there is a quite significantly sized second set of DLC, but none of it is essential to get into the game, and many of the newer DLC features are now standard in the base game - see the recently released Tanks expansion for an example. The vast majority of the enormous number of user-created mods and missions for the game only require the base game itself, so this is pretty much the ideal way to jump into the series if you're at all curious.
The Humble Arma 2018 Bundle is available here, and will be live for the next 13 days, at time of writing.