the "best" build depends on the situation and you playstyle...
im assuming you want info for pvp.
as hammer warrior you could work as a normal tank, or try to work as an interrupter...
the "possibly good" ones
-aftershock is nice. much damage for only 10 mana if the enemy is knocked down (wich should be no prob for a hammer wielder

).
it is a very good addition to your knockdown skills, giving a great boost to your potential damage output.
however to be effective you need a relatively high earth skill.
and if you only want to take one elementalist skill with you this might be a waste.
-armour of earth is really good. a long duration buff for only 10 mana.
even at earth 2 it gives already 29 armour class. against everything! thus reducing the taken damage by more than a third.
it is good for tanking, but the speed slowdown makes you pretty... slow. :P
so a caster running away will get away from you.
-magnetic aura can give you some short time defensive boost, to use a signet of healing for example.
however i find it too expensive (or too short) to be of great value.
(and if you tank, earth armour is IMO much better to absorb the -40 AC of the signet)
-ward against melee is much better in that role.
for only 10 energy, you get an un-dispellable, area protection.
it GREATLY reduces the damage output of any melee enemy. not only do they hit less often, fewer hits also mean less adrenaline-> less special attacks.
even at relatively low level of the earth skill this has a good duration...
AND it can benefit your whole group.
AND there are many warriors out there that dont know about its effects, and will continue pounding... at maximum 50% of their original damage output.
AND as already said, there is no way for your enemy to get rid of it, except by avoiding the ward.
-obsidian flame is a dangerous spell.
exhaustion is nasty, even moreso for a warrior/ele.
the damge output is not thaaat great at low earth skill... i wouldnt generally use it as W/E.
but if you already have a high earth skill (for one or two other earth skills) this is a nice finishing blow.
no other skill has the ability to "garantee" such ammounts of damage in a short time (only monk protection spells reduce the damage).
the "bad ones"
-earthquake and eruption are IMO of no use for a warrior.
too costly, and too long to cast them.
-crystal wave is obsidian flames big brother, without the exhaustion.
however it is quite costly too... and you dont want to be a caster anyway.
(in PVE this is a different story though...)
-same goes for stoning. even if you inflict weakness, the mana=effect ratio is just bad.
-iron mist and gasping earth are only of use if you have another char doing other kinds of damage.
-kinetic armour is pretty useless for you, as casting spells constantly makes you a bad warrior.
the "depends on... " ones
-stone daggers...
hmm, hmm... normally a warrior should not use a spam spell such as this, especially as it is only of limited value against heavily armoured foes.
however, it can be a nice "ranged attack" to finish off fleeing casters
(esp. if you have earth armour on you).
you need some points in earth magic for this, but no need to max it.
-ward against elements gives you and your group some elemental protection...
hoewver it is 15 energy, and much less effective then ward against melee is against melee attacks.
if you only want to protect yourself, earth armour is much better.
-ward against foes slows the enemy down.
useful in some arenas...
but it will not offset the earth armour slowdown, as it has only a limited area of effect.
==>>
if you only want to put a few skillpoints into earth, i would advice taking earth armour and ward against melee, making you a MUCH better tank at the cost of mobility.
if you go for a "medium" ammount you can choose between the earth armour and/or ward against melee and/or stone daggers, as tanker with ranged attack.
or go for aftershock and/or obsidian flame and/or ward against melee, as more of a damage dealer.
i would not advice maxing earth, as you dont have the mana to use all the skills anyway.
6-8 points at lv 20 are enough to make most of the cheaper spells worthwhile for you...