>>6693380The Bandai Model Kit in 1/144 scale is definitely the only option. Has all the articulation you could ask for in an AT-AT.
You notice how complex the big leg joint looks right below the belly? With that moving beam next to the big circle joint, moving along the groove? Or the aforementioned
>>6694380 turning mechanism that enables the legs to turn slightly outward? Both are only in this kit.
Dragon Molds is working on a 1/35 Scale AT-AT that should be comparable in articulation but the last thing I heard from that was one or two years ago.
Don't get the big F-Toys AT-AT, I got it recommended here and it turned out the legs use hexagons instead of circles to snap the leg joints together, making it a statue when it could easily have had working joints.
All other options I found like action fleet or micro machines are always very lacking or plain statues, I am amazed how bad the options were until the Bandai kit got released despite the extremely simple design.