I won't be posting mine in Fan Designs only because it's too simple, upgrading to DHS does allot. Staying on topic and keeping away from fan designs, the 12S does allot more that many GMs wouldn't allow without a official RS.
The 12S gives us the firepower of the 3S, upgrades it while hardly compromising any survivability for speed (if you loos both side torso on this mech...how? ) The 5S does a great job but that IS XL can be a problem (yes, I'm talking from personal experiences. Always load Infernos) The 6S, 7S and 9S are decent compromises between the 3S and 5S (the Wobbie 8S isn't to be ignored, just a different design direction going 100% AX Murder)