Coming into E3, the top-billing in the first-person shooter genre clearly belonged to Microsoft’s Halo 3, followed by Sony’s Killzone 2. But after E3, the first-person shooter that everyone raved about was Activision’s Call of Duty 4: Modern Warfare.

While I had some trepidation regarding developer Infinity Ward’s decision to bring the game out of the 1940s and into today, those concerns dissipated after the game stole the show at E3. Today, they’re completely gone.

I’ve been playing the multiplayer beta (which will be released to the public on Monday, August 27) on Xbox Live all week, sharing countless games and conversations with many of the game’s developers, and I’ve come away convinced that even the omnipresent Halo 3 is going to get an online run for its money from the latest Call of Duty.

The beta is simple and straightforward, with the ability to find a game, visit the “Barracks” or invite a party to join you. Much like Halo 2 did, your party can stay together from game to game, making for a better experience with friends.

The standard “Team Deathmatch”, “Free-for-all” and objective modes are available, including the “Domination” (hold certain points on the map) and “Search and Destroy” (collect a bomb and plant it at a target while the other team defends) modes. Free-for-all isn’t my bag, but the objective modes are challenging and fun, and even the team deathmatch mode provides an adrenalin-fueled blast of mayhem.

The game itself is gorgeous and runs at a slick frame-rate even with 16 players online at once, although the servers aren’t as flooded as they will be after the game releases, of course. The controls are well-tuned and easy enough for a novice to use, although the game doesn’t borrow from either Gears of War or Rainbow Six: Vegas and use a cover scheme, which is a bit disappointing. Over and over again, I’d want to hide behind cover and “stop-and-pop”, but in this case, Call of Duty 4 isn’t as tactical as it looks. It’s more in line with Halo or Unreal Tournament - it’s meant to be fast and furious, with non-stop action, and camping - anywhere - isn’t recommended.

Unlike those two games, however, jumping around incessantly like a caffeine-addled kangaroo isn’t terribly advantageous, and thank goodness for that. Nothing removes you from the excitement of a game faster than watching heavily-armed soldiers bounce up and down like kindergarten students at recess - and it’s the simplest way to separate the men and women online from the children. Instead, the “jump” command is really more like a “clamber” command, allowing your soldier to quickly get over and beyond obstacles in his way.

The online multiplayer experience is about achievements - no, not the Xbox 360 ones - yours. Everything you do has value, and helps to make gameplay rewarding and enjoyable. Every “kill” you get is worth experience points, as are - finally! - assists. If you didn’t deal the final blow (usually earning 10 points), you’ll five or two depending or how much you helped bring your opponent down. Planting a bomb (whether it eventually gets defused or not), taking down helicopters or performing other tasks will also grant you experience points, which increase your rank. You’ll earn additional points in team games (even when you lose) based on how well your squad performed. As your rank grows (in the beta, it’s currently capped at 11, but on the menu, it extends to at least 55), more options for customization will be available to you.

After you’ve reached the fourth rank, you’ll be allowed to create a class, choosing your primary weapon and secondary weapon and adding any accessories you may have earned - more on that later. You’ll also select three “perks”; special abilities that your character possesses (IGN lays out which perks are in the beta here - more will be available in the final product), and the combination of the three helps your character become unique.

You’ll earn weapon accessories, like scopes or camouflage patterns, by proving your proficiency with each weapon; but not the type of weapon - each particular model. Reach a certain number of “kills” with the weapon, for example, and you’ll earn a new scope, silencer or other fun widget that can be added to your customized weapon; combined with the create-a-class, you’ll have the ability to create five new classes that perfectly fit your playing style and be used at a moment’s notice.

For example, I created a sniper class with a high-powered scope, a silenced pistol, two RPGs (for those pesky helicopters), the UAV Jammer perk to stay hidden and Deep Impact, a perk that allows my bullets to rip through soft cover. Needless to say, that’s one dangerous dude - unless, of course, he’s in battle at close range; in which case, he’s toast - you might as well put the controller down.

After earning an extra grip for the SAW machine gun, I chose to add that for greater stability and accuracy when creating a new class of heavy gunner. Give him some flash grenades, the Juggernaut health perk, and the Steady Aim hip-firing perk, and you’re ready to go “Rambo” on the bad guys, but you won’t be outrunning anyone, and hiding certainly won’t be your strong suit.

The flexibility here is obvious, and it’s sure to provide a gamers with a lot of different things to try.

Hopefully, there will be many customization options for your online alter-ego, a la Rainbow Six: Vegas (in the beta, there are none). It’d be a shame to have so much flexibility in the weaponry and have the same four or five soldiers wreaking havoc on the battlefield. Call of Duty 4: Modern Warfare’s online play is about making you feel unique, and here’s hoping that the game delivers fully on that promise.

The three maps included in the beta give you an idea of what’s to come. They’re complex, especially the Crash level, and are large enough to accommodate big groups - as well as force you to keep moving. Gameplay seems to be well-balanced, as no weapon seems to be “the best” - although snipers with quick reflexes are still supremely dangerous - and the clever radar system works well, on the whole. Players will be visible on the enemy’s radar when they fire weapons, unless they’re silenced, and the fact that there were players in the beta with silenced assault rifles made me a bit nervous about the possible exploitation of that system. Regardless, it works like it’s supposed to at this point.

If you go on a run, offing three enemies in a row, you’ll be able to call in the UAV, which will reveal all the opponents on the map unless they have the UAV Jammer perk for roughly 15 seconds. Get the streak even higher, and you’ll be able to call in an air-strike, which sends everyone on the map running for cover; or an attack helicopter, which will cruise around the map mowing down your enemies until it’s dispatched by a rocket launcher.

Speaking of rocket launchers, explosive weapons always seem to make people edgy in shooters, and many turn them off entirely. It’s my feeling that this won’t be necessary in Call of Duty 4. Grenades can be picked up and thrown back, and grenade/rocket launchers are limited to two shots in total with no reload capability. And possessing them keeps players from taking a perk or other grenades. I think Infinity Ward has solved this balancing issue - there’s still the threat that you might get a rocket in that window you’re sniping from, which keeps things interesting, but the game can’t devolve into an insane frag-fest. It feels just right.

Since the game doesn’t release until November, there’s certainly much more to come from Call of Duty 4, but what’s in the beta now impressed me greatly - and far more than the beta for Halo 3 did. This isn’t to say that one will be much better than the other - it’s not like Halo 3’s unimpressive, mind you - but for a game that just recently landed on gamers’ radars, Call of Duty 4: Modern Warfare looks like it could be the best experience on Xbox Live this winter.

Now, if you’ll excuse me, I have to run - I owe that developer who kept sniping me a little bit of you-know-what…