Carr's Champion Tup but really you would be best with something without feed value such as willow branches, cow parsley, carrots and apples if they like them or even very small bits of digestive biscuits, a piece each. Soays in particular and wethers in particular again, don't need an actual feed treat. Both Soays and Shetlands can get too fat quickly and can become pushy and rough with you from hand feeding. It's very cute when they eat out of your pocket but the shoving and jumping can be too much. Just because they're actually wethers doesn't mean they don't think they're tups.
I agree too that treats a couple of times a week is enough to keep them friendly. We give our males a very small amount of Tup&Lamb coarse mix when it's snowing or frozen solid, otherwise they graze and browse the hedgerows, with ad lib hay in winter and willow branches.