7mm STW by far. It doesnt require special dies, loads of load development and is very load friendly. Not knocking Kirbys brilliant line of mags but they do require a special amount of work to get to shoot right. The 7mm can be a finicky caliber when extreme overbore is encountered. This may not be what your after.
The 7mm stw only downfall is its a belted mag and to some the belt equates to less accurate. While this may be the case I have yet to see a difference and have enjoyed years of accuracy with my couple of 7mm stw's. In a 34 inch barrel behind 74.5 grains of H1000 I was able to get 3100 fps using Berger's 180 grain VLD's. This far exceeded the 7mm Ultra factory offered heaviest bullets in a shorter barrel burning more powder.
The ultra however if chambered in this same barrel would probably offer 3300~3350 or so this would be a substantial benefit and worth it. If your end product is within 100 fps of each other the difference between them at 1000 yards is hardly noticeable but if you plan on going out to 1500 and beyond then something else may be needed. Anything out to 500~700 yards is all velocity beyond that is BC and velocity.
Sometimes in the quest for the best you end up spending more time in the garage/shop and range than you do hunting or enjoyable shooting. Spending loads of time and travel trying to find the perfect combo of powder, bullets, and group size can be overwhelming.
A lot of these magnum wildcats require some load development. What the fellas on here are getting didnt happen with one load and a trip to the range it tooks some months to find the right combo.
The stw if chambered in a 1:9 twist barrel by a good barrel company will like powders in the range of the H1000, Retumbo, US869, RE22, RE 25 with bullet weights in the 168~180 class with great results. Loads of data exist on the net and on here and you can usually get a few loads ya like and they'll also perform the same in your weapon as they did in the others.
I enjoyed the 7mm STW and its accuracy as well as its forgiveness in small loading differences. I am currently upgrading the wifes gun to a new Bartlein barrel in a 1:9 twist Sendero contour finished at 26 inches fluted 5R rifling. I know it will be a screamer and I dont even have the barrel yet. I thought about the 7mm WSM, the 7mm Ultra and yes even the 7mm Allen mag.
I want a fun gun right out of the starting gate and I know this will do it. Most of the other cartridges when you get into 100 grains of water capacity are a waste unless you intend to extract every ounce of performance. Huge cases require longer barrels to take advantage of all that powder and that means heavier guns unless you want to dump $800~$1000 in an ABS wrapped barrel at which point you should go with one of the 100 grain capacity cartridges as money must be no object