Well for a good tank build, even alistair, you will need a nice set of gear and a proper build.
For me i use at lvl 17(not that high): STR 42 /DEX 36/CONS 25/WILL 20
Gear: Well you can make him to be a anti mage tanker, but you will kinda lack the good stuff for holding large hordes of enemies with any healh problems. For an Anti mage you can use the knight comander plate, boots of diligence, gloves of diligence and the beren's hornor as helmet
+5 Willpower +6defense +4 armor +2armor
+40% Spell Resistance +2rmor
+10 Mental Resistance
For the shield: Havard Aegis for the spells resist and some chance to ignore magic
For the sword : Any one handed sword with 3 slots (starfang , topside honor, keening blade are good choices, and the 3 deomer runes will provide +30% spell ressist.
For rings: Key to the city (+2 all atributes+4 spell resist t) and livegiver/rings of ages/harvest ring are good choices also for more hp/reg, resists or stats.
Amulet: The spellward (+30% magic resistance), 10% dodge atacks and other stuff.
Well with this gear alistar will be imune to almost any kind of magic, but its not really that good with hordes of enemies since it's made from silverite and the set itself dont give any really powerfull bonus, mostly because the armor.
My favorite tank build is more a regen hibrid build with a magic resist weapon/shield on offset and dmg/defense set for the main weapons for any kind of trouble. The idea is being able to replenish hp and stam in a incresead rate with a good protection.
Cailan's set: for regen, dodge bonus, fatigue reduction.
Cailan arms: more regen, a good def and incresead dmg(3 grandmaster deomer runes or 3 elemental or silverite runes).
Spellward: +30% resist, 10% dodge and other stuff.
Rings: Key to the city and lifegiver *great regeneration*
Prolly your tank will be almost immortal, works really good if you main char is an archer or a thief. You will kinda have an huge amount of health and hp/stam regen, +64% spell resist, +30dodge atacks, a high defense due to the 35-45 dex and increased def and armor with shield wall.
For second specialization reaver or champs do the trick, reaver will keep you hp up because of the devour skill and aura of pain for an area dot (the regen is so high that negates the effects of the self dot) and for the last the frightning skill gives you one more disable if you are up vs an ogre or any other enemy with high physical resist. While Champs boost your party defense and attack, really good for a tank.