I just wanted to offer another point of view. I've been playing a magician since 2000, so, while the learning seems endless, I do have quite a few hours logged on this class.
At around level 4-8 I used the earth pet a bit, but I didn't like how little damage he did to the mobs. At these levels, I was doing a lot of fighting alongside my pet, so I didn't need mobs to be rooted off me. In fact, if they're rooted, the mob can't run at low health, so we'd get no freebie hits on mobs' backs (and they were still hitting me and the pet until the bitter end.)
I'd read that water pets were a good all-around pet so I switched to them primarily. Good damage, second in HP to the earth pet, fastest regen, almost immune to poison. Yup, they're a good pet for soloing.
If I was in a group where we were fighting casters, I'd switch to the air pet. Those stuns are awesome for interrupting casts, but the low hp on this pet made it dangerous to use for soloing. If the group was fighting non-casting mobs, then the fire pet was great for mad damage output.
When I hit around level 30, I decided to give the fire pet another try at soloing. My fighting tactic switched from simply nuking the mob while the pet tanked to alternating nukes and pet heals, due to the fire pet's lower hp. The fire pets tore up the mobs and I used them quite a lot up through the level 49 version. The votes are in and the post 49 fire pets has been judged utterly useless. I agree. /mourn Sparky
The post 50 pets are modelled after player classes:
Air = Monk (excellent at avoiding hits & keeps mobs stunned motionless through a good portion of fights)
Water = Rogue (backstabber and still a good all-around pet but takes more hits than the air pet. Very good for duoing and grouping. Your best choice if you solo by aggro kiting)
Earth = Warrior (still a lump, as far as I'm concerned, but the generally acknowledged choice for chain petting.)
Fire = Wizard (but unlike a good wizard, he makes bad casting choices and has a pathetic mana pool. Melees about as well as a wizard.)
At this point I switch my pet choices depending on what and where I'm fighting. No one pet is good for every situation. The key is to learn how each one excels.
Every magician has their own preferences for fighting and this will color their opinions. My soloing preference has been mainly in outdoor zones, single pulling mobs and killing them as quickly as possible. I, like many other magicians, don't do a lot of chain petting. In my mid fifties I switched to grouping and duoing mainly. Just those factors put the earth pet very low on my list of best pets.
I just wanted to point out this last part because there really is no "best pet" answer that applies to all of us.
"God created man from a handful of dust, and he created woman from that man's rib. And these two together were so stupid that they weren't on the planet five minutes when they managed to get a curse put on all future generations. Nice work." Pat Condell