Team with okita and scathach

A good support would be Alexander because of Charisma, his 3rd skill gives a party wide Quick buff that lasts 3 turns and he has an AoE Quick NP that gives 15 stars at base overcharge. But he is also a 3 star with low attack and low hit counts.
Until Skadi, the only servant with strong Quick buff besides Alex is Atalanta, she also has a Quick AoE NP and can make Scathach even more nuclear while also generate stars. But she has awful NP gain and a Lancer, Saber and Archer comp can cause problems.
Both of those have high star absorption but both Okita and Shishou have absorption on one of their skills so you can circumvent the fact that these can steal stars from them.
I recommend Andersen because he is a Caster (won't steal stars), can buff crit damage, has 3 arts cards, can give them passive heal, can passively generate stars and also raise their attack and defense with his NP.
I suggest you roll for Skadi since she is amazing and makes those already great servants inro destructive forces of nature.

People will say Alexander but he is a Rider who has a high star gather rate.

- For consistent Crit playstyle, Hans equipped with any Crit star gen CEs and Jack is also a good ways to gain Crit star and Crit more consistently. HNS is pretty good, similiar way goes to any Crit Up CEs.
- For swift kill or OTK tactics, Helena and Mozart or Blackbeard is one of the best for it. Prioritize CEs with high Crit damage Up and high Atk stat or any of your best Offensive CEs.
- For harder content or challenges, Waver and Merlin and Mashu and also bring some tank or taunter. Survivability CEs recommended.
- For almost all kinds of situations for your Quick team, Caesar is the best of the best for them. Atk Up and NP damage UP for 3 turns with just 5 turns CD, along with very strong Crit buff with short CD. He's just the right man for ya !
Don't forget his Bond CEs will increase your frontline party members Quick card by + 15 % as long he's still alive on the field.
Atk Up + NP Up + Quick Up + Crit damage UP + Crit star drop up, what else do you need ? He's a complete package of a total steroid and that's why Caesar is Caesar, being fat doesn't mean you're not powerful.

Bonus : Scathach Assassin herself can quite make some Crit stars, combined with her Quick card decks and a taunt plus targetable heal and damage cut she's a pretty good Quick member for sustained battle.
Fuuma is also another amazing Quick member, with targetable Evade and strong capabilites of making the Crit star himself.

My best Quick team yet : Kintoki Rider ( main DPS ) + Helena ( main Quick Support )+ Assassin Scathach ( Taunter and semi-support ) + Caesar ( Steroid buffer ) + friends Scathach + OG Chu or Marie Rider on the backline.

One of my favorite Quick set up, Kintoki ( Gem Magecraft/Knights Dignity ) + Helena ( Crit star gen CEs ) + Hans ( Crit star gen CEs ) for fast kill and Crit everytime. I use Knights Dignity crit damage Up + 40 % for my Kintoki to increase his destructive power.
Combined with Hans Crit damage Up + 40 % will make my Kintoki into a beast of gameplay for almost any general bosses, sometimes Kintoki will ignore Class advantage system like Saberlot does ( Consistent Crit + 80 % Crit damage up, very good to quickly destroy things ).

I mainly use Golden Sumo or HNs for Kintoki for the first team since he can receive 4 times on the battlefield and different buffs stacks multiplicatively ( 2x Quick buff from Kintoki himself, Quick buff from Scathach Lancer, Quick buff from Helena ). Too much Quick buff is not good, adding another buff is a better choice to do. I use Chaldea Combat Suit for this set up.
Kintoki Rider himself is one of the most powerful and broken rider on NA, he'll become more powerful once Skadi arrives.

Quick team is really2 fun to use and their crit consistency is the best yet on FGO, don't forget to save for Skadi if you're serious about building your Quick team.

Alexander for more quick support and attack up, but he's a 3* Rider, will rob you of most stars, and has low hitcount.

Caesar, he has the same deck as those 2, NP and attack buffs to make them more powerful in their burst turns, even more crit up buffs to add to their own, and huge stargen up buff. He is saber, not so many stars are lost, but he also has low hitcounts and is 3*s.

Wu Zetian, to be released in the Agartha Sub-Singularity, also has that rare AOE Quick up buff (sadly it does not included herself, who is a quick servant), Assassin 4*, so she does not take away so that many stars and is more powerful. She can cast one target def down.

Helena, buffs all cards for 3 turns, including quick. AOE NP charge, from 10-20%. Caster deck means more chances of NP charges, but less stars (she also has a passive stargen up with horrible cooldown).

Blackbeard (extra heal for females, Voyager of the storm) can be a decent support, if you overlook his rider starweight and low stats.

Waver and Hans, the universal supports.

Scath a has horrible NP gain, put some star gathering CE on her (Runestone, Fragaratch, Seal Enforcer...), so her quick and arts crits will happen far more frequently, filling her NP bar. Or just starting NP gauge CE. Also, ABB brave chain fills more NP than AQQ, but you probably noticed that.

Okita can work with you either putting crit up CEs (Atumbra, Rin's Pendant) to make her even more critter (and not needing to waste the evade), or quick up CEs (Immaginary Around, Gandr, Innocent Maiden...), for a even more consistent dmg/stargen/NP gain from her good quick cards. Or you can Black Grail/Heaven's Feel/Angel's Song/DragonKin on her to multiplicatively stacks with her Quick Mana Burst, for a massive NP dmg.