There are hundreds of servants to choose from in the Fate/Grand Order and thousands of them are viable. All 5 star servants are exceptional, but some are better than others. The rating of 5 star servants is based on how flexible they are, how powerful their skills and abilities are, as well as whether they can function well with other units.
The 5 star servants, as well as the most powerful, are the rarest in the game. There are some lower units of rarity that can compete with them, but generally, for tougher tiers, a 5 star servant is your best bet. It is difficult to get them, especially if it is on a free account to play, but it is possible to get a few with event banners and proper Saint Quartz management.
5 S Tier
These servants are contenders for best unit in the game:
-Scathach Skadi implemented new Fate/Grand Order tactics by making fast units far more successful. The power of quick cards and also their critical damage can be buffed by her. If that’s not enough, through fast cards, she can also increase NP gain, dealing huge damage in the process. Her biggest drawback is that she doesn’t have such a strong HP and defence, so she dies quickly. However, this is not a major concern because she shouldn’t need to be durable, because she can destroy enemies so easily.
The power of Merlin comes from his absurd team buffing abilities that make it a walk in the park for all fights. He improves the capabilities of buster characters, passively generates vital stars, and has good boot damage. His importance is evident in the end game and mid game stages, regardless of the difficulty, as a help.
-Zhuge Liang is a fairly straightforward servant for all allies with the ability to increase NP gain. His other buffs have really high stats, so during a tough fight, they’re perfect for buffing a team. As a S level servant, if he’s available, he should be included in almost every squad.