Well, most Isekai's use a similar letter system. (G),F,E,D,C,B,A,S,(SS)
With some differences in what's required to get a higher rank.
For example in
Reincarnated as a Sword
[ul]* G-Rank: Newbie with a provisional license. Not an actual adventurer.
* F-Rank: Apprentice. Beginner adventurer.
* E-Rank: Average adventurer who can proclaim themselves as such.
* D-Rank: Mid-rank. Can be a party leader.
* C-Rank: Veteran. Superhuman by civilian standards.
* B-Rank: Top class. Usually the strongest member of a small guild.
* A-Rank: Hero. Select few within a given country. Bards begin singing songs about you at this rank.
* S-Rank: Legend. Only right have ever existed. Kings bow before them, and they have the authority to command Guild Masters[/ul]
But among those isekai stories I can't recall many where the MC is eager to rush to the highest rank. Mostly the Guild just assigns a higher rank after stuff happend. Or the MC is OP from the start.