Well that partially depends on your skill level and how you like to play. It's kind of like comparing fruit when you compare these games. Sure apples, pears, and mangos are all fruit and pretty similar, but there are major differences as well that prevent a true comparison.
But as it is to answer you question, provided you are talking about the online multiplayer only in my opinion:
Gears of War takes the most skill by far. The online games rely heavily on teamwork to be successful and its hard to do well if you just have one or two really good players on your team when you are at a competitive level. The controls are also slightly more complicated and the gameplay a bit slower, requiring a more tactiful approach rather than run and gun. Also there's no killstreaks or special power ups. Just power weapons.
Halo takes the next most skill. Now don't get me wrong...Halo is the easiest of the 3 to just pick up and play. It's even the easiest to be semi-decent at without much skill. BUT to be GOOD at Halo isn't that easy. You have to master aiming and timing to get those DMR shots down. At a highly competitive level missing even one headshot can be your doom. When playing Pro or "MLG" style Halo also offers one of the most level and fair competitive fields among FPS competitive gaming today. That means you can't use many tricks to be better than the next guy, you just have to be better. Now true in standard Slayer online there is cheap crap like Armor lock and jetpacks, but other than that...
CoD comes last, and i know ppl are going to rage bout this. But any idiot can pick up CoD and blast off a killstreak, call in a helicopter, and rack up a bunch of kills. All it takes is a little luck. Now, yeah you die easier in CoD...but that means you kill easier too. If you just happen to see someone and ur not a terrible shot general it just takes a few bullets to do the job. Not to mention CoD is a campers dream. Anyone can rack up a few kills if they decide to camp the whole game and no game does that like CoD. And let's face it...lack of a "MLG" playlist means deathstreaks (which are completely retarted) and killstreaks are RIDICULOUSLY overpowered. Not to mention spawning with any weapon you choose puts a power weapon in the hands of everyone all at once. Trust me...more kids play CoD.
If you don't believe me rent all three games and play them online yourself and see which one u do best at. 1000 internets says it's CoD.
Now if you're talking about single player then going off the hardest difficulty setting i'd say: CoD, Halo, Gears. Opposite lol.
