Best saber in the game?

Go for Rama, Caesar or Fergus. Caesar and Fergus are 3* even.

Sabers are good all-arounders, and many of them specialize as AoE NP nukers. The problem is that not many stand out too well.

Okita is absolutely fantastic on a quick team, Nero and Shiki are good on arts teams. The rest of the 5 stars are just the aforementioned AoE nukers. Out of the nukers, Altera is probably the best.

Rama is a great burst damage dealer with his ability to absorb stars and good critical damage. As the other guy said, Fergus is a good budget AoE nuker if you don't have a 5 star. Ceasar is a fantastic support for many teams.

Siegfried is garbage, but I still love him :(.

If you are not p2w then it is not like "which saber should i roll?" it is "woah 6th Siegfried? Thank you so much". Basically you will just have to love all sr/ssr you will get and fill in other slots with budget 1-3 servants, luckily this site provides with in-depth guides for these guys. If you haven't been burning blindly all 1-3 than you will not have much trouble.

Nero is the best in question to survive and do a lot of damage or have a great defense
And its 4*