Focus on what you're good at! If you are decent with most guns breadth is a great way to gain some quick levels. Otherwise, use you're favorite gun and mop up the competition.
Ranking up like a bad ***
Gun Challenges
Its also worth noting that more kills trumps completing challenges almost every time. What I mean by this is don't spend your time on a gun you suck with for experience. The opportunity cost of the kills you didn't get by using a crappy gun greatly outweighs the experience you gain from the challenges. By using a better gun you'll get more kill streaks which in turn gives you more experience and more kill streaks, and kill streaks also give experience!
Kill Streaks
Pick low kill streak rewards (as these provide you with extra experience every time you call them in) and try and get these often (hardline?). Keep in mind that getting an osprey gunner, while awesome provides THE SAME AMOUNT of experience as calling in a uav. By using low achievable kill streaks you will gain more experience by using the kill streak, gain more experience by challenges for the kill streak, and gain more experience by the kills you get when your kill streaks are up! Most of this game is about snowballing your momentum.
Equipment
One of the most often over looked ways of gaining experience. Not utilizing your equipment is bottom line a wasted opportunity. Don't think of throwing that flash bang grenade as -wasted- think of it as utilized. Every time you die with out throwing your flashbang you have wasted the opportunity to potentially save your life, potentially get a kill, and work on your equipment challenges. Wasted equipment is sad equipment. Utilized equipment is just one more layer of the snowball (see above)
Keeping these things in mind will help you level up faster, but its worth reiterating the most important part of this description. If you are using crappy perks and crappy guns to get challenges YOU WILL LOSE EXPERIENCE. The smarter you play (low kill streaks, great perks, great gun) the more kills you'll get which will lead to more kill streaks which will lead to more challenges which will lead to more experience!
Thesis
Now this seemed like a long detour to the point, but here it is. If you play better you will level up faster.
As far as an 'optimum route' this really depends on how you play. Ever had a friend argue with you for hours about how 'their xyz' is always the best but it just doesn't work for you? You and I will be significantly better with different guns and different perks, but you wont find whats 'optimized' for you until you try it. Try it out on a 'test' class while you get a feel for which guns and perk choices fit your play style best. Want to try a new gun? Go for it. Just make sure you always dedicate a class to something you know you can kick ass with. That will give you a fall back while you investigate which perks and guns are good for you!
Hope this helps
MW3's new point streaking system means numerous actions now count toward your killstreak rewards; plants, defuses, captures, defends, kills, and even assists (with Hardline Pro) count now. Destroying killstreak rewards (helicopters, SAMs, turret guns, etc.) will count against your killstreaks, for every strike package.
These do not count toward your actual kills (in terms of K/D ratio, leaderboards, game limits, etc.) - only toward your killstreak rewards.
Best Answer
The variable zoom scope is not essential but it is a big plus on the larger maps. Other useful attachments are extended mags if using a semi-automatic like the Barret 50 cal or the AS50 (bullet spam can make up for bad aim on occasion). If you don't want to have to camp and snipe a lot another strategy is to equip the ACOG scope on one of the above rifles and use it more as a long range assault rifle, with the Kick proficiency and the ACOG they can be remarkably stable and accurate.
In the long run the best way to level up a sniper rifle is simply to practice with it until you get good enough that you can use it on most maps so that you are always racking up XP with it. Using it with an ACOG will make it more versatile and usable on smaller maps but on smaller maps you're more likely to take a shotgun to the face so probably not the best of ideas unless you are a lethal no-scoper.
The other benefit of using it all the time is that you'll learn all the crazy sight lines and get better at quick scoping which are both important to getting more kills and thus more XP.
Personally I predominantly play sniper and the only maps I don't snipe on are Hard Hat, Dome, Carbon and Arkaden because they are too compact and busy. Maps like Output, Bakara, Mission and Fallen are probably the best sniping maps but if you're a beginner with the sniper be prepared to get taken out a lot by the experienced snipers until you've learned the sight lines and ways to move around the map out of the enemy sniper sights.
Bootleg, Lockdown, Resistance, Seatown, Underground and Village are all good maps for practicing sniping because they have some good sight lines but are less prone to being dominated by expert snipers like the aforementioned maps. Again for these mid size maps ACOG scope would work pretty well.
Edit
So I think the main difference between variable zoom vs ACOG is your preference for play style. Variable zoom scope will give you really long range kills but you need to be good at holding your breath and aiming, ACOG is super stable but shorter range. It works better if you like to move and snipe while variable zoom is better if you prefer to stay still and move infrequently.
Perk wise I use Sleight of Hand Pro as it is useful for fast switching to a machine pistol as a close quarters weapon. If you want to move around the map a lot (or camp in the open) blind eye may be useful to give you immunity from air support.
Secondly I use Hardline to get my killstreak faster, if you prefer like me to snipe from the same place for a while and move around infrequently it can be hard to rack up large kill streaks as enemies will often come flush you out of your favorite sniping nest. I usually go with Assualt Strike Package, either:
For the final perk either Marksman for the longer range enemy identification plus Marksman Pro will give you longer hold breath when you get it. Once I unlock it I usually switch to Dead Silence which is useful for sneaking around to flank enemies without them hearing your footsteps.
Equipment wise I usually have Portable Radar as my tactical (I unlocked it with a prestige token using Unlock Gear so I get it immediately rather than at level 75) which helps me spot enemies trying to sneak up on me. Paired with sleight of hand this usually gives me times to swap to my machine pistol at short notice.
For lethal either bouncing betty/claymore are useful for booby trapping an entrance to a sniper nest or just dropping in an inconspicuous place on a well travelled route to pick up extra kills.