the KT is probably the lightest and slimmest, but i haven't handled a GSG... but i do know how thick a MP5 is. most KTs seem to need some tuning, at least to sand down some of the flash on the plastic. the KT has an aluminum aftermarket trigger blade available from Red Lion, and several companies make rails for it, but no drop in improvements for the trigger.
there's also the now discontinued S&W 15-22 pistol. light, takes the 10, 25 or BDM drum mags. takes any standard AR trigger, so the trigger is upgradable. i have a 15-22 rifle and really like that. was disappointed that they dropped the pistol
the Charger has everything available for it that works for a 10/22. trigger, bolt and barrel upgrades, stock upgrades, (well, swaps, i like the Charger stock, but it's not very packable) just don't put a buttstock on it without the paperwork. i have a Charger that i'm just starting to tune up. the Charger needs to be pulled from the stock to really clean the guts, same as any 10/22. not a major problem, but more work than a KT or S&W. the charger comes with a bipod.
the GSG looks like a pain to clean from the videos i've seen.
you don't say what you want to do with it, is this going to be a pack gun, varmint plinker or just a range toy? if you want accuracy, the ability to upgrade the Charger with goodies to improve the accuracy is a plus. if you want packable, the lighter weight of the KT and S&W would be a plus.