Various XP questions :

[Saevel]

1. What is the best use of my rested XP? With speedrunning PoTD the go-to for XPing nowadays, I have a lot of the bar I can't seem to use up.

2. I've heard that you run Great Gubal Library from 60-61...is that the new Aurum Vale of HW and SB? Does it really give that good XP?

3. Also heard that PoTD is best until 60, and then the SB dungeons for XP?

1. Kill monsters that give you exp, in dungeons and the open world. Rested bonus is not applied to rewarded exp (fates, hunt marks, POTD, etc).
2. Gubal is fine, so are fates in the new zones.
3. POTD is a fine choice, depending on how long it takes you to clear a set of floors. The reward is not insignificant.

Above all else, remember that leveling is a rate: exp earned over time spent. Get all the rewarded exp you can in the time you have, then maximize that rate.