Depending on your group and the type of mob, consistently fight things 5-10 levels above you. Again, kill time to down time trade off. Sometimes you will find that lower level mobs actually give more exp and are fast to kill because of the star system.
I see a lot of posts that recommend fighting mobs 5-10 levels above you. I disagree. This strategy used to work really well before Update 3. Since the update I find it best to fight 1- and 2-star mobs my own level plus or minus 2. Granted, I'm a druid. Other classes might be different.
Why did it work before the update? Because you got an xp premium for fighting above your own level. Defeating a mob significantly higher level than you used to have a huge payoff. I don't remember the numbers, but I think it was somewhere around 25-100% xp bonus depending on the level difference. I used to see level 30 warriors in shalemont power levelling on level 45-55 mobs with an out of group druid all the time because of this.
Why doesn't it work now? The upshot is that tougher mobs take longer to kill, require longer recovery between fights, and you don't get an xp premium for killing them. The xp bonus for killing a higher level mob is much lower than it used to be. For example, a 1-star golem my level nets 2500xp when I solo it. Add a level to the mob, and that xp goes up to 2560. Add another level, and you get 2620. But while the xp payoff goes up a hair, the difficulty goes up significantly. Mobs hit you harder and evade more of your attacks. Meanwhile, your auto attack and skill damage on higher level mobs is lower than on same or lower level. Remember all the low level drop fishers in your Shivercowl group taking 60 fire cloak damage and dealing 5 points? Well, this is pretty much the same scenario, applied to levelling. A mob 5 levels higher can take as much as 50% longer to kill than one your own level. It was worth it when the xp premium was 50% but not now that it's closer to 5%. Hardly worth it to fight higher levels unless you are starved for mobs.
Why kill lower level mobs? Returning to my druid player vs golem mob scenario, I'm level 126. I get 2500xp from a 1-star level 126 golem. I get 1960xp or so from a 125 golem, and about 1500xp from level 124. This is faster than the 5k I get from a 3 star my own level.
The only exceptions I can think of are if you are in a duo or group situation and can kill reasonably fast. Or if you know the spawn points of the new Wisdom mobs. In which case it's important to always kill the normal mob on the Wisdom spawn point to increase chances of them spawning.
The other thing to consider is difficulty of killing mobs of the same or similar levels. Level 53 Connacht Ribbonswords have really low hp and poor armor and are easier to kill than some level 48-52 mobs. They're also much easier than boars. Black golems have less magic resistance than grey and brown ones, so are easier for druids to kill.
Also, consider whether redoing your skills with a book of alt when you outlevel one zone and go to a new is worthwhile. For example, you might be a fire mage for lower shalemont but it could serve you better to be an ice mage versus magma golems. (Note, that was a random example to illustrate my point. I haven't played a mage so have no idea if this specific case would make sense.) I do know that as a druid, taking a handful of points out of my damage spells to put into abundance made a world of difference for the 7 levels I worked that zone.
