"Best bag" is tricky, because the best one is the one that does what you need in a given situation. For me, it's a dead heat between the C-bag, which is large enough for work trips and A4 papers but small enough to be slung over my shoulder when I walk down the aisle of a train, and my custom front rack (since refined and productionised by Kinetics) that carries my Carradice Super C front panniers, which don't do A4 but also don't need waterproof covers when it rains. I once adapted my old
Halfords handlebar bag to fit the Brompton luggage block; practically speaking it worked fine, but it was too small and too low down.
The UPSO bags are neat, though. They're made in the Carradice factory, which is why they use their fixing system.
The best front bag might actually be one you already own, hooked onto a Brompton bag frame. I've used a touring saddle bag in that way before, and would really rather like Carradice to make a Super C version of their Stockport bag.