Every character, regardless of class, starts with a base of 20 in each of the non-deflection stats, and gains +3 every level, so at level 16 they have a base of 65 in each. Deflection varies with class, with the Fighter starting with 30, Chanters and Monks with 25, and every other class with 20 or less. In particular, Paladins start with 20. Deflection gains +3 with every level like other defences, so again at level 16 a Paladin would have a base of 65 Deflection (a Fighter would have 75).
However Paladins have a special ability called Faith and Conviction that automatically activates when combat starts. This gives them a bonuses to all defences dependent on their reputations. With 3 or higher in both of their favoured reputations and no points in their disfavoured ones this talent gives +11 Deflection and +22 to all other saves, giving Paladins an effective base of 76 Deflection and 85 in all other defences. This is the reason Paladins are considered to have the highest potential defences. As far as I recall, all other bonuses to defences are available to all classes and hence can be taken by a Paladin, so they can maintain their lead on other classes if they wish.
Onto more specifics of how to build a super defensive Paladin: focus your attributes on Might and Intellect, followed by Resolve. Dexterity should be your dump stat (if any), keep Perception at 10 and Constitution no lower than 8. This gives you high Fortitude (the most important defence) and Will defences. You will then take Weapon and Shield Style and use a shield to raise your Deflection and Reflex saves even more. You definitely want to use the Outworn Buckler for an effective +5 to all saves as well. This will probably raise all your defences to around 105ish at level 16. Beyond this point you could take some or all of the defence boosting talents, but honestly doing so is probably overkill. With gear and spell buffs you'll already be at the point where you can tank every encounter in the game so you really don't need all of them. Perhaps the Fortitude one is worth taking, as is perhaps one of the situational ones that add +20 to specific types of attacks.