I'm going to convince you that Tierno is the best friend character. To do that, I would firstly contend that each different friend in Pokemon X/Y represent a different aspect of the Pokemon fandom.

Serena/Calem represent the competitive side of Pokemon. They want to win, and win badly. Their whole character arc involves learning to accept that they mightn't have the skill they'd like to fulfill these ambitions, but that's true of many competitive battlers - for every person that wins a major tournament, or have the top rank for online ranked battle, there are many underneath that person that also wanted to win but lacked the skill and ability to do so. Yes, Serena and Calem aren't very good, but then, who ever said that talent and ambition were one and the same?

Shauna is a "fun" battler. She'll use her favourite Pokemon, whether everyone else likes them or not, or she'll use a certain gimmick, even if it's not the most sure way to win a match. Her second battle with you, on route 19, and she uses her starter with a default ability, plus Goodra (currently an OU Pokemon) and Delcatty (last gen, was the NU of NU, unofficially called the PU tier). If she wins with her Pokemon, she'll be quite happy, but she's not as obsessed about winning as Serena and Calem are. It's not merely winning that's important, it's also the team she uses, and how she plays to earn that win that are of equal, if not more importance to her than winning itself.

Trevor is the Pokemon collector. He's the sort of person who would own collected every form of pokemon like Vivillon, Burmy, Wormadam or Florges. That one day plans to have a living Pokedex, or even collect shiny Pokemon, regardless of their quality. He might battle occasionally, but would much rather be collecting or breeding pokemon, even if they aren't the most competitively viable pokemon.