I've had success with high-dex polearm "fighters" of various classes- PsyWars, Monks, Fighters, "War" Clerics, etc., and multiclass variations thereupon.
The basics:
Your PC needs to be proficient with a decent polearm or reach weapon (duh), have a dex worthy of taking Combat Reflexes, and then follow up with feats like Stand Still*, Hold the Line, Deft Opportunist, and so forth.
Refinements:
- Spells like Enlarge, powers like Expansion, racial abilities like Powerful Build, and feats like Monkey Grip or Heavy Weapon (depending upon the particular polearm) increase your base weapon damage.
- At some point, some opponent will close within your Polarm's effective range. There are 4 basic ways to get around that- using a reach weapon that can be used against adjacent foes, Improved Unarmed Combat, Armor spikes, and Shorten Grip or Short Haft (2 similar feats from different sources). Races with multiple limbs have an additional way around this- wielding a conventional weapon in addition to the reach weapon.
- Some races make better "reach warriors" than others, due to unusual stat adjustments (Githzerai and Anthro Apes have a +6 to Dex!), or other racial abilities (Half-Giants with Stomp & Powerful Build, Thri-Kreen with multiple arms = multiple weapons, the Githzerai Inertial Armor psi-like ability). Don't sneeze at that Human's extra feat, either.
- Some reach weapons are better than others, so choose carefully. Guisarmes, for instance, give a bonus to trip attacks and are only martial weapons. Greatspears (my personal fave) are 2D6 reach weapons that can be thrown if need be, but are exotic. The Kusarigama (another fave), doesn't do much damage, but can do all 3 damage types (depending upon the writeup), and as a chain weapon, can do those tricks as well.
- Some PrCls are better than others. The Kensai can be brutal- with its inherent ability to enhance its chosen weapon and the +8Str "Power Surge" (which stacks with all kinds of bonuses) can tear opponents up quite nicely.
* Stand Still appears in the XPH, but it is a general feat that also appears in another WotC supplement (which escapes me at the moment). It allows you to make an AoO to make a foe stop moving.