Which rider should i play?

Santa Alter will hit the hardest because she is a free np5 with a 45% mana burst. just level her first. The other two aren't very good.

Marie: Cu style rider. Top as durability, mediocre for others

Santa Alter: Saber alter in rider version, with NP5. Performance is close to SSR level. She is also good candidate as single DPS for Rider Critical team if you dont have other SSR rider, or Ushi.

Trap: No urgent to level and use her until come of Skadi.

Go raise Santer first, you'll use her very often and overall she's a solid Rider. I really recommend Santa Alter, because Astolfo and Marie kit/servant build isn't as straight forward as Santa Alter.
She can do your wave clearing and farming job at a faster pace than Astolfo and Marie.
And her main point is, her NP in terms of damage is almost equal to an ST NP in comparison.

Marie is a turtle, her best job is to stall, solo battle, as an anti-male rider and also for harder content/challenge alike.
Can also act as your anchor/last woman standing vs Caster, Berserker and some pillar enemies.

For Astolfo, DW need to fix her soon.

Astolfo is broken, he can solo Goetia and it's a massive bro.

He also is a Trap God: https://www.youtube.com/watch?v=YMvjj002hJE

Go for salter for dps and marie for tank, generaly you want to have at least 2-3 rider for upcoming solomon temple singularity since you'll fight a bunch of demon pillar

Being a free NP5 servant, Santa is great for farming since her NP is AoE. As if the damage wasn't amazing already, she gets a nice buff to her Buster cards from her second skill, and has great damage potential through critical hits from her first skill and from her natural class advantage in absorbing critical stars. Make sure to get all her copies and ascension mats before the event is over, otherwise you'll face the biggest problem of welfare/free servants: They'll get stuck on a certain level since you won't be able to raise their level cap. So don't hesitate to buy those chicken buckets and all her copies from the event store. Or believe me, you WILL regret seeing her stuck on lv 40-50. She deserves more that that, and so do you.

Marie, on the other hand, hits like a wet noodle since her base attack and lack of offensive skills truly hamper any chance of her doing some meaningful damage. However, she compensates for that by being an amazing tank, and a nearly impossible one to kill given her high HP and her skillset, composed of heals and invulnerability. She's also useful against male enemies with a somewhat unreliable charm, but that can really come in handy in challenge quests. It's also worth mentioning that her NP is not just AoE, but also heals and removes debuffs from your entire party. In other words, she can be an amazing tanky support for difficult missions.

It is no exaggeration to say that Astolfo is terrible, and easily one of the worst servants in the entire game. Unless you really like him, I can assure you that you'll get a lot more from burning that shitty trap into rare prisms than from wasting valuable resources trying to make him work. He doesn't fit into any niche and doesn't particularly excel at anything. He's only 'useful' if you need a rider to kill 1000~ HP Caster minions, but honestly, even 2-star riders like Edward can do that better than him. Either burn him or let him gather dust in your second archive.

Salter. The other 2 are quick focused and are harder to use well.