Is the 22 mag the most powerful 22 round made?
What are some of the other 22 calibers outthere.

Nope.  The 22 WMRF is adequate for most small to medium game, but it isn't the most powerful .22" round by a long sight.  That award would go to one of the several centerfire wildcats that push velocities well over 4000 feet per second.

Quoted: Nope.  The 22 WMRF is adequate for most small to medium game, but it isn't the most powerful .22" round by a long sight.  That award would go to one of the several centerfire wildcats that push velocities well over 4000 feet per second.

Who makes rifles chambered for the wildcats?
and how much do they go for?

'Cartridges of the World' by Frank Barnes will  give you a good description of each of the commonly available cartridges.

Cost varies - you might have to get a custom rifle built by a gunsmith for some of the rarer wildcats.  If the round is rare enough, you'll also end up paying more or handloading your own ammo.  Say a couple thousand dollars or so.  On the other hand a basic .22LR might fit your requirements for fifty bucks including a box of ammo.  At the higher 4000+ fps velocities barrel erosion and per-round cost become factors you should consider.  22LR is popular and so inexpensive it's almost free.  22 WMRF, .223 Remington, 22-250 and 220 Swift are also popular .22 chamberings with progressively more power as you go through that list.

Edited to please grammar teachers everywhere.

If its a rimfire the mag would probably be the faster and most powerful.  But if you are looking for sheer speed in a 22 cal there is a firm overseas that does some really wild stuff.  Last I heard they were up to 5400 FPS.  They use solid copper bullets
 http://gscustom.co.za/