If you're focusing on implement attacks and not going for anything crazy-optimized, don't worry too much about what weapon you're holding (if you're even still holding one). There's a handful of decent weapon enchants which give extra healing bonuses, most of which are mace-centric, and you've got a baked in shield bonus with BCL, so you don't need to restrict yourself to one-handed weapons, either.
If the question is what /implement/ type is best for you, that's a bit harder to answer. The short answer is staff because it has the best expertise feat, but the big catch here is that clerics aren't proficient in anything other than holy symbols natively, so to make that work you'll need to MC into a class that has staff implement proficiency. MC feats are pretty strong anyway, with the proficiencies being a free side-bonus, so this isn't an unattractive option. At level 1, though, my optimized answer would be to just take holy symbol expertise, and at level 2 to retrain into MC invoker for 1/encounter hand of radiance for minion-popping and for staff implement proficiency, and to then take staff expertise.
In general, weapons that are also implements (like staves and daggers) are more optimizable, because you can use the weapon-specific enchants that change your damage type to a specific type of your choosing (frost, fire, radiant), apply a matching dragonshard augment for +1/3/5 damage, AND still get superior implement bonuses. Radiant Weapon is also one of the easiest ways for an implement user to obtain an item bonus to damage.
From a crazy-optimization standpoint, the best templar is actually a hybrid invoker dizzying mace morninglord, in my opinion, and this combo pretty much requires using a crusader's mace (and works nicely with Mighty Crusader's Expertise). But if you're not going for Morninglord, the radiant damage type doesn't help you that much, and if you're not going for dizzying mace, the MC fighter and mace-as-implement doesn't help you at all (and you don't need to hybrid in order to still have access to MCing fighter for dizzying mace).
tl;dr: staff, with caveats.