Which job seems the most 'relevant' to the main storyline?

[Enkidoh]

I agree, Summoner definitely is a strong candidate, because it's confirmed in the Job storyline that in order to summon a Primal Egi and bind it to your will, you have to have defeated the Primal itself first, and the scientist in the storyline actually called upon you to help with her research BECAUSE you had beaten Ifrit already.

The main problems with Summoner being canon:

1. I think you very likely would have to keep your practice hidden, in order to avoid being thrown out by your city state.

2. Since your first 30 levels are as an Arcanist, it would pretty much confirm that Limsa Lominsa is the only canon starting city - because it just happens to be the city you spend the least amount of time in during the Main Scenario.

As for Warriors and Paladins, my own personal issue: I'm sick of seeing fate grant melee classes the world on a silver platter, everything that we spell casters have been toiling our whole lives to achieve!

But I would say any of the Jobs would work, strictly speaking, depending on what city you started in. It's confirmed at many points that the PC is not the only Adventurer who is going around trying to help the world, so the way I see it is, just like in a single player Final Fantasy game, each Job storyline correlates with a specific Adventurer who canonically had a part in defeating the Primals.

Ironically, in regards to 'Derplander' and his party, in 1.0 at least, all indications were that he at least was Limsan Lominsan - the original 1.0 opening movie started in what the Drowning Wench was actually planned to originally look like (complete with aetheryte in the centre of the room - early on it was decided not to have the aetheryte inside the Adventurer's Guilds for obvious lag reasons), the guildleve card he holds clearly says 'Limsa Lominsa' on it, the 1.0 benchmark program featured the opening cutscene/event for a Limsa Lominsan player in 1.0, and even in ARR, the 'Crystal's Call' trailer showed him meeting with Merlwyb and also at the Maelstrom Command giving the Maelstrom salute!

Of course this is all circumstantial at best, but it's as good as proof as anything really.