Of the above scopes mentioned I have owned the Kahles 525 SKMR3, Tangent Theta 525P GEN2XR and Minox ZP5 MR4. I still own the Tangent Theta.
I personally would not recommend the Kahles. Unless you absolutely love any of the SKMRs, need ambidextrous controls, or focus at 20m.
The Minox ZP5 is excellent. Especially at the used price point now in the PX around $2400-2500. I really think the ZP5 is the best value scope out there. The glass is excellent, MR4 is about perfect, turrets are good, and parallax is amazing. Here is the link to my Minox ZP5 review:
Minox ZP5 5-25x56 MR4 - Initial Impressions - "Pleasantly Satisfied"
The Tangent Theta's cost premium is mainly going to be an upgrade in the turret department. The turrets and toolless zero are unreal. The Gen2XR is really growing on me and in use is an outstanding reticle. From memory of the ZP5, since I no longer own it, I honestly can't tell if the Tangent Theta has better glass, I would need to have them side by side. The parallax, meaning parallax free past 300 yards, is similar, but again I would need the side by side to compare.
I have no experience with ZCO and I am anxious to get my hands on one.
In summary I have a hard time recommending the Tangent Theta GEN3XR for a $1,000 premium over the ZCO527. At the $4k price point between the TT Gen2XR and ZCO527 the decision becomes much more difficult. Having said that the differences between the ZP5 and TT525P are small and if you are cost minded I do not think you will regret the ZP5.
For what it is worth I have a review of the Tangent Theta 525P Gen2XR CCA that should be posted by weeks end.