Probably depends on why you want one
But here is my nickel....
I don't like the physical package of the MX5. I don't see why they have to continue with 8 AA batteries when it's already been proven that you can get the job done with 4 AA. The plastic battery box itself is not to well designed and is prone to fail. (I have a full line Whites dealer in my city so I get to handle and play with them all and that battery box latch is a real pain) Also, with any auto ground tracking system, a
must have feature is the ability to "lock" the ground setting. The ability to disengage the tracking system, so that the last ground balance setting will stay in place until such time as you wish to change it. The MX5 doesn't offer this, which means lots of false signals and noisy operation. The only way to disengage the ground tracking system on the MX5 is by using the pinpoint function, and that is only in place to keep you from tracking out the target when pinpointing. I'd recommend you run away and never look back at the MX5. That said, I'm sure there are fans who have low mineral dirt that love it and I just tee'd them all off
I don't like the Outlaw. This is just a personal preference. I like tone id. The Outlaw doesn't have tone id.
That leaves the F5 and the AT Pro. I like both of these.
The thing I like best about the ATPro is that you can hunt it sssssssssssllllllllllllllooooooooooooowwwwwwwwwwwlllllllllllllllyyyyyyyyyyyyyy. (that spells "slowly" for the ADD afflicted folks). No joke. You literally hunt the ground one inch at a time with no loss of performance. The iron audio is great too. Small display but after a little while you would get to the point where you set it up and just hunt by the tones and forget to even look at the display. Great with EMI interference also. Its a good detector.
The F5 is just a great all round detector if you can learn to drive it. A lot of features packed into the F5 package. I've got a ton of info already written about the F5 you can look up. I kept the F5 over the ATPro because I get more bang for the buck out of the F5 as compared to the ATPro. (also I live in the desert southwest and don't do any water hunting where I'd need to submerge a detector).
So between the 4 listed, I'd look at the F5 and ATPro and see which one fits your hunting style.
Good luck.
Mike