The great aspect of being junior on the 747 is that most trips are still within reach. The 9 day BKK/SYD is a senior trip but on a blindline you may get allocated one, but chances are reasonably slim.
Other popular routes are SIN, PHX and the new LAS. However, most of the guys on the 'bottom page' have been to these destinations, albeit infrequently.
The real crunch for junior pilots is how much do you value weekends off? If you are prepared to work all your weekends you will probably get 'better' trips. But then one mans great trip, e.g JNB for golfers, is another mans dross.
High credit 3 crew/3 day trips are popular, SEA, YVR, LAS.
After time on line, you will work out what trips you like. There really are very few trips that are very unpopular, maybe MEX is not high on most peoples list.
As I said, you'll probably work most weekends to get the more popular trips, but you will still be able to fly almost all of the network.