The Polearm is the best weapon. It also has reach but much better damage and you can combine it with Polearm Mastery and maybe Sentinel.
No it isn't. A Polearm is 1 point of damage per hit better than a Whip and dueling and you have a worse AC which means you are more vulnerable to taking hits and losing concentration.
Further unless you start with an 18+ strength, if you use 2 feats with PAM and Sentinel you are doing over 1 Damage less per hit then you would be with a Whip since your strength is 4 points lower (and you are missing more often). Even when you add in the bonus action attack at 8th level per round it is 12-33 (avg 22.5) PAM using a bonus action with a 16 strength vs 16-24 (avg 20) without using a bonus action and a 20 strength. That is if all of them hit and doesn't account for the worse chance to hit.
Also, the main reason you want reach is Wrathful Smite, and that conflicts with your PAM bonus action.
You do get reaction attacks from PAM and Sentinel and that makes up some damage but not enough.
I am not saying that PAM+Sentinel is bad, but it is not generally optimal for a Paladin under 11th level. Wrathful Smite is one of the best 1st level spells in the game, it is almost as broken as the Shield spell when used with a reach weapon. That is why they are changing it in the ONE. On a Paladin the most effective way to use it is with a Shield, and therefore a Whip (unless you are a bugbear).
When you hit 11th level and enemy saves become better, legendaries become more common and you add improved divine smite things change, but below 11th level it is not generally optimal.
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