Recommended Servant

Scroll up you’ll see the Tier list, enjoy yourself.

Look no further than the 1-3 star tier list for good suggestions. There's no servant that beats all but the tier 1 and 2 servants are all solid. Mash and Hans aren't necessarily good, but they fit in all team comps with powerful, relevant buffs which makes them very valuable.

For high rank, the most generalized support would be Waver and Merlin. For 4-stars, Helena and Gilgamesh are probably the ideal support options. Hercules is also a very good 4-star Berserker.

Caster: Hans for support. NP5 Hans, max all his skills, full ascend him. Budget Waver/Merlin. And Medea for ST dmg (but put a NP dmg up on her, like Angel Song, Heavens Feel, Dragonkind; her NP is very weak) and raise her 1st skill to 4. There's no good AOE low star Caster, but go with Cu Caster in the lack of better options, as he's though, or Paracelsus, for minor arts support.
Shakespeare and Mozart first skills are good for nuking, but they are weak; max those skills, but no need to fully max them themselves.

Rider: Ushi for ST. Do her NP interlude. NP5 her. Extremely good boss killer. BlackBeard for AOE, Voyager of the Storm is very useful tool of support dmg, put buster up CE on him. Georgius for defensive support, 3 turns taunt.

Berserker: Lu Bu is the only ST Berserker. For AOE Kiyohime or Darius are good options.

Assassin: Cursed Arm Hassan and Kotarou give you many stars, Kotarou is AOE and debuffer, CA is ST and has protection from the wind, good in surviving longer. 100 Faces is also good, her 1st skill on a hight level makes her both a good stargenerator and NP gainer.

Saber: Caesar, just look at his kit of offensive support; also the only ST Saber you will get in a long time. Put quick up CE on him. Fergus for AOE, but he has 3 busters, will be hard to charge it twice; do have a Dragons Meridian on him allways.

Archer: They are all amazing, do focus on Euryale and Robin a little more.

Lancer: FSN Cu Chulainn for ST, Romulus for AOE. Put def up CEs on them.

All i need is your advice about best servant in this game.

Simple. Merlin - Supports are King, he's the King of Supports.
He's simply broken, and far to powerful of a servant. Lacking any offence, but who cares about that let someone else do it.

Cheap servants? Reroll your account over and over until your first 4 Star is a Herc. He's one of the strongest berserkers, always helpful (even was in the Solomon story).

Make friends with a wide variety of people who have a variety of supports. Double or triple up on them for CE changes to suit yourself.

Always go for who you like, not what's best. Always. You will enjoy Fate far more with your waifu / husbando then you will just going for the best. (if who you like is best, even better!.... mines not ;-;)