The BNet Avengerdin
There's a lot of good Paladins who have been enjoying the key skill that takes out nearly any boss, regardless of immunity or resistance. The Avenger. Many of these excellent players enjoy the use of the powerful and very obvious aura, Conviction. While I still use this, I prefer a different approach, one that can very easily make your Paladin a great deal more helpful to party.
There are two builds for this... one-handed Avenger, who has a Shield and the bread-and-butter skill to go with it for Block Rate % and Defense, and two-handed Avenger, sacrificing those meaty shield-blocks for pure unadulterated damage.
First, the Skills
Combat Skills
This tree will differ most depending on your build. For the most part, the Paladin's aim will be either to solely max his Vengeance, or to increase both his offenses and defenses with Vengeance and Holy Shield.
Sacrifice: No Prerequisites. 1 Point.
This will be your early boss-killing stroke, at a minimal health cost in Normal Mode right up to Level 18. Of course, only one point as a prereq for your key skill.
Smite: None if two-handed, 1 Point if one-handed.
Smite, the awesome stun-bash with your Shield, can be powerful with the right build. That's not our style, though, so just take a point down the road if you plan on sword-and-boarding.
Holy Bolt: 1 Point, only if one-handed.
Holy Bolt's shining days as a low-cost, high power bolt of undead slaying might are gone, even if it's gained a minor healing ability. While fun to play with, only one point as a prereq is needed.
Zeal: Sacrifice required, 1 Point.
Now, most Avengers probably place a good 4 points, 2 or 3 minimum, into Zeal as their group-attacking, drain-sapping quick attack. With the right gear, you'll have enough +skills to get the full 5 strikes with one point in it as a prereq for your big gun.
Charge: Smite required, 1 Point only if one-handed.
Another shield-only skill, Charge sends us flying into battle. While certainly useful to the Paladin as a way of getting to the forefront of combat, and for pesky runaway duelists, we only need one point as a prereq, and then only if we're using a shield at all anyway.
Vengeance: Zeal required, 20 Points.
Vengeance is our key, high-power skill. Dealing normal physical damage imbued with three seperate elemental modifiers, each stroke of this attack deals immense chunks of pain to the enemy. We will be maxing this one in time, though not too early, so as to keep the mana cost low.
Blessed Hammer: Holy Bolt required, 1 Point only if one-handed.
This radial attack sending out spinning anti-evil hammers of holy power are powerful indeed, but they're only interesting to us Avengers as a prereq for our sacred defensive skill.
Holy Shield: Charge, Blessed Hammer required, 1+ Points only if one-handed.
The defensive boon of all Paladins who wield a shield, this skill raises their defense, block % and smite damage heavily. Of course, this is only useful to a single-handed Avenger, but is quite useful all the same, so placing leftover points can be a good thing for shield-wielders.
Defensive Auras
Surprise! This tree is where a large number of the true Avenger's points will be invested. I will only be listing the skills which will actually receive any attention from your skill points.
Resist Fire/Cold/Lightning: One for every 6 levels, and each necessary. Every 2 hard points raises your passive Max Resistance to the respective element by 0.5%, whilst every hard point invested when one of the auras is -active- gives a 1/1 ratio Max Resist boost. Great for early boss packs with elemental damage, it also raises the damage of your Vengeance through Synergy. 10 Points into each should be invested before beginning to raise Vengeance likewise.
Salvation: While a late skill, Salvation provides both the buffs of a table-wide resistance bonus whilst active, and a table-wide elemental synergy bonus to your Vengeance. You can max either this or Vengeance first.
Prayer: I suppose if you're two-handed you could spare one point into this skill, if only to get the next step up.
Cleansing: The only weakness of this particular Avenger is a lack of natural defenses to poison, especially if you decided to skimp on resistance gear. This alleviates that greatly, while helping your allies, removing poison and curses much faster. Nobody likes being poisoned.
Meditation: While it's a stretch, you might like to nab a point in this too, if you don't decide to just pour points into Defiance for non-elemental bosses. Casters like Sorcerers and Necromancers will love you for it, and you'll get a nice mana regen in case of large skeleton packs (no mana stealing).
Offensive Auras
Amazingly, none of these will be required, for one simple little reason. If you're playing Ladder (as you should be, since this is a BattleNet Avenger guide), invest the time in getting a good socketed pike/lance/war lance for your Act 2 Merc. Defiance, Might and Holy Freeze are all exceptional choices for your needs, but whichever Mercenary you choose, you're going to give that lucky monster of a minion the Infinity Runeword. This grants that hireling a natural Conviction Aura while he lives, giving you some incredibly high damage and hit rate.
Alas, I'm not much of a gear-nut just yet, but that's the basis of my guide. Get your Vengeance, pump it with Resist Auras and Salvation synergies, and then just either blow the rest in Holy Shield or get some nice support auras. Once you get Infinity onto your merc, you can confidently take on near-anything that Hell's got to throw at you. Keep Sorcs, Druids and Amazons company, too. They'll love you.