Synergy advice

Herc have him in all your team he’s that good

Herc+Hans+ support Waver or Merlin and you’re basically set for life in this game

My suggestion is to prioritize Herc and Mash first, they are best units in their rarity. Then you might want to raise some popular names like
-Archers : Robin, Euryale (best nukers), Billy (Critter, the drawback is his lackluster decks and generation). David(semi-carry, very useful in hard content)
-Lancers: Both Cú are great for surviving
-Riders: Uwashikamaru or waiting for broken welfare Kintoki this summer
- Assassin : Unfortunately, low rarity assassins are not great to be main DPS , i suggest you to roll for King Hassan in his rateup next weeks, best Assassin in game.
- Caster : For utility, Han’s best.
Team composition: Hans + Mash + DPS, but generally you’ll need plenty experience to figure out the synergy of the team.

Put a servant with class advantage and some supp or tank, that's should do the job.

Art team, David/Robin/Euryale/Georgios + Hans + Mozart/friends Tamamo/Waver. Robin for OTK + Debuff team and David for harder quest/challenges.

Buster team, Hercules + Hans + friends Waver/Merlin/Shakespeare, like mentioned above. Lu Bu if you need some damage or want an OTK ( One Turn Kill ) team. Shakespeare for a turn Burst.

Quick team, Kojirou/Ushiwakamaru/Billy ( Crit magnet CEs, Alex can steal stars LOL ) + Alexander + friends Scat/Atalante/Helena. Waver if you need a faster NP gain and borrow Kintoki Rider if you want easier gameplay.

All team is amazing, it's up to you to use them wisely.

Bonus :
if you need a Buster Crit servant, Guda-Guda Honnoji re-run will coming soon. Archer Nobunaga Oda is a pretty strong Crit based servant.
Cha-Cha, AOE berserker to help you dealing with multiple class enemies/bossess.
Kintoki Rider will arrive when Onigashima re-run is coming, a broken servant and top tier Quick and ST rider.
Hot Bikini ninja ( assassin Scathach LOL ) from summer re-run.

There are lots of pretty good welfare servants to catch this year, good luck !