As far as I know, the German plan was to take Antwerp and the supplies there , then make a beeline for the channel and cut off a whole lot of Allied troops.
I could be wrong, but I just don't think there were enough Allied supplies to capture to keep the offensive going for too long. That said, I believe with a little luck the Germans could get close to Antwerp. They would never take it since the Allies would pull every available soldier possible to defend their one major supply port that wasn't rubble. They fight in the city and the Germans are in trouble. Like Stalingrad showed, you can't blitz through a city and they would bog down, take far to many casualties, and run out of supplies.
Then they run into MAJOR trouble. Trouble spelled P-a-t-t-o-n. OTL it took him 2-3 weeks(?) to turn and counterattack the German flank and relieve Bastonge. The German objective is Antwerp and no one at this point is going to disobey Hitler, especially if they think they are on the verge of a major victory. The Germans will need everything they can get to take Antwerp and that leaves Patton (or Monty if you're a Brit sine no one can agree that both could do something productive simultaneously) with very little in his way.
The German "bulge" gets cut off and is completely screwed. The western Stalingrad, you could say. So the war ends up ending earlier with Germany loosing a huge number of irreplaceable equipment and men (many of whom are the best Germany had left)
Edit: Or just don't attack, but I assume you meant the best outcome of operation Wacht am dem Rhine, not the best thing the Germans can possibly do at the end of 1944. Besides, by this point the Germans had a propensity for NOT doing the best thing.