The best of the best, these servants significantly improve your team. Not only are they the most effective at their role, but not including them puts you at a disadvantage.
These servants are among the best at their role. They are strong in most generic teams, but perform best when their team is built around them.
Solid servants that can perform their role reasonably well. They may have one or two weaknesses which need to be compensated for by support servants.
Servants that are only viable with a class advantage. They may be useful in specific instances, but there are better alternatives.