Polearms are good for control, not damage, unless you are a Dragonborn with high Strength, or a level 16 Iron Vanguard
As I have written elsewhere, there are 3 good reasons to use reach weapons, and a bad one.
Damage is not really one of them. Polearm Gamble is great if your DM loves you or absolutely lacks any tactical sense, but if this is not the case you will never use it.
Paladin is one of the worst classes for a Polearm build, as Brian Ballsun-Stanton explained on the same link, they lose a lot by not being adjacent.
Fighters also need to be adjacent for their punishment, but at least they get easy access to the great Polearm Momentum feat and lots of pushing and sliding powers.
Wardens can pull off some great tricks with a Polearm and a Fighter multiclass, but they need to hybrid for a pushing at-will power.
Barbarians with a Fighter multiclass and the Savage Reach or Aggressive Lunge power are probably the class that benefits most from polearms and does a decent damage besides. Consider the Novice Power feat to take Rain of Blows with a Greatspear.
Only a special build of Slayers are good polearm wielders. From Paragon, you can create a revenant half-elf with Eldritch Strike as an at-will power. They are crippled from levels 1-10, but after that they are devastating.
Leaders generally benefit a lot from polearms. As they are not so resilient, want to stay away from the bad guys, but still want to be able to hit with melee powers. Warlords, Aritificers and Ardents are all great, but none of them excels at striking.
As accuracy is more important for average damage than the base damage of the weapon, you should go for the Greatspear. Spears also have good feat support, so unless you intend to use the weapon as an implement or need the Heavy Blade Opportunity, you should pick Greatspear over Glaive.
Polearm Momentum is great for control, and with Draconic Arrogance or Iron Vanguard it even adds damage. You might need a Fighter multiclass for this.
Impaling Spear improves your accuracy quite a bit if you use Melee Basic Attacks.
Surprising Charge is probably better with a Gouge, but a Greatspear is also good.
Spear Push is good to get the push 2 needed for Polearm Momentum.