It depends:
The highest killstreak reward you can obtain is different for the three strike chains, with one final hidden killstreak for all of them.
For the Assault Strike Chain, the Osprey Gunner is the highest, requiring 17 points without Hardline, and you will be in an osprey gunner for 75 seconds, delivering Care Packages to a preset location.
For the Support Strike Chain, there are three different ones at 18 point, which are:
The EMP, which destroys all enemy electronics, disables electronic attachments, disables the HUD for enemies, and causes everyone to have a slightly blurred view
The Escort Airdrop, which is basically an Osprey Gunner with you on the ground, and the Osprey is AI controlled
And the Recon Juggernaut, where you are equipped with Juggernaut armor, which gives you a huge health boost, and are majorly slowed down, with a Riot Shield and a USP .45
And for Specialist Chain, the Specialist Bonus (8 points), which gives you every perk, and proficiency (except for Damage and Attachments)
And finally, for every strike chain, is the M.O.A.B., nicknamed the Mother of All Bombs, or formally the Massive Ordnance Air Blast. It requires a player to get 25 kills with none of the killstreaks kills counting, which makes specialist bonus the most popular one to use to obtain MOABs. The MOAB has a small countdown when activated, and when it finishes, a massive blast occurs, slowing time down, until it kills the entire enemy team, causes an EMP, and leaves the map shrouded in red dust.
By Completing challenges
Your fourth kill streak slot is for killstreaks aquired from care packages.
Unfortunately no you can only have 3 kill streaks equipped at a time.
Because in C.O.D MW3 all of your kills count towards kill streak rewards. For example if after you killed the right amount of people to unlock the pred missile you can unlock the next award in your kill streak set with kills gained from using the Predator missile. Hope I helped.
To get the title 'toxic'in mw3 you must be killed by an enemy MOAB. A MOAB is a giant bomb you get in mw3 for getting a 25 kill streak. Or 24 if you are using the perk Hardline
46
you have to get a 25 GUN streak preferably using specialist package so you have no kill streaks and with hardline 24.
You have to use the assault strike package in order to get it. (it's a 17 kill-streak reward)
53 kills
The highest prestige in MW3 is 10th. 10th Prestige level 80.
Call of Duty 4: 7 kill streak: Attack Helicopter Call of Duty World at War: 7 Kill streak: Attack dogs Call of Duty Modern Warfare 2: 25 kill streak: Tactical Nuke Call of Duty Black Ops: 11 kill streak: Gunship or attack dogs.
Kill kill kill !!