Which direction is best to hike the Montana Trail 406?

Westbound is the least popular as you are paddling against the current of the snow melt Missouri River in spring/summer. Westbound does provide an early opportunity to start on the eastern prairies and provide time for the snow to melt.

Eastbound thru-hiking is the most popular and has its advantages as well as disadvantages. In the mountains, you will probably experience snow-covered trails until late summer and hot and dry conditions on the eastern Prairies. Eastbound hikers should be especially aware of the potential hazard of late spring and early summer snowfalls and choose a start date carefully.

Flip-flopping may be a good option for thru-hikers that prefer to avoid snow travel in the mountains. Flip-flopping requires starting the thru-hike near the middle of the trail at Fort Benton as described in the question above.