I have a 17.2 hand 1/4 draftie. I have tried more blankets on him than I can remember (including Dover's Rider International) and what fits him best are Rambo XL. Unfortunately, it seems they aren't being made anymore, so the next best are the Amigo and Amigo XL (cheaper anyway ) For him, the Rider's Int. did fit ok, and has large shoulder gussets, but just was NQR in some way (I think the neck opening was still not quite big enough for him).
Until recently (when I guess his shoulders got bigger?), the old style Weatherbeeta Orican (bigger gussets, not the Freestyle model) fit decently.