Definitely the bus. It's the most direct. Shouldn't take longer than 20 minutes once it arrives. We take the monorail when we want to relax and sometimes it can take about an hour total.
Ok, I'll be honest though, unless we are going to MK we almost always drive. It's the perk of camping and having my car right there. When I am at other resorts I always use WDW transport but it's so easy to drive.
We really do get spoiled at the Fort. The only time the buses really seem full is at night and it seems like only the internal buses. We have never been on a full fort bus any other time. This past november we stayed at CBR and holy crowded busses.
We were at the Fort late November 2014 and had to take the bus from MK to the Fort (boats weren't running for some reason). Well..... that was a trip. Had to stop at Wilderness Lodge and change over. Waited at the lodge for about 30 minutes for the bus. The entire trip from MK to the Fort was about an hour....