A few of these matchups fit into the category of pitting the tricksy against the reliable. Makes for some interesting decisions between them:
Loragorn vs. Na'asiyah - Na'asiyah's ability is a more unusual one where Loragorn's is very simple but also very potent.
Haldir vs. Bard - Both are good Ranged attackers, but Bard's ability is a straightforward better at ranged attacking while Haldir offers timing shenanigans and staging area hits.
Legolas vs. Timrahil - Legolas is the classic Tactics hero, been around since the Core and very solid, while Timrahil offers new and exciting (if not perhaps as dependable) tricksy play to the Tactics sphere.
Beregond vs. Caldara - If you wanted to pick out a hero as the epitome of dependable, you'd struggle to find better than Beregond. And equally it's hard to get tricksier than Caldara.
Boromir vs. Spirit Merry - As Rajam said, Boromir is the classic OP but boring, whereas Spirit Merry enables Secrecy decks and other odd shenanigans.
And then there are the matchups I'm actually finding difficult to pick for which also somewhat fit this pattern:
Sam vs. Hama - Hama's tricky, Sam's pretty straightforward and reliable. As I said in the first round of voting I've been enjoying Hama more lately, but equally who doesn't love Sam? I might pick Hama in the end just because I think by most people's perspectives he'll be a bit of an underdog.
Elrond vs. Theodred - That extra resource every round from Theodred is super reliable and powerful, but then on the flipside Elrond buffs healing, smooths resources between spheres and can use Vilya. I kind of want to go Theodred because he's so good and somewhat underappreciated, but Elrond wins out majorly on cool factor.
Gandalf vs. Leadership Gimli - Off the top of my head I'd say Gandalf is Caldara's only real competition for the title of 'Tricksiest hero in the game', whereas new Gimli offers a powerful but straightforward ability by contrast. Gimli gets a definite advantage from being shiny and new, and the ability definitely appeals, but I love some of the shenanigans-y stuff you can do with Gandalf and I know that even after two years I've still barely dipped a toe in the vast ocean of possibilities he offers.