system
Given the following post, respond as if you're commenting on Reddit:
human
[General] What universe is the easiest (or most difficult) to start as a low rank and work your way to a commendable rank?

For example, in FMA universe, everyone can learn alchemy but it'd take years of studying to do it. So, if I was an ordinary person, I'd take a few months to years to practice alchemy and get the hang of it, then a few more years to hone my skill. Depending on how difficult it is, I may or may not reach the required skill to become a state alchemist however.

Another example can be Konosuba. Register at the guild, pray you have a decent class, do quests (push through the low supply of low level quests) or do other objectives that can help progress you if you have an adventurer class (same class as Kazuma), level up, get stronger. Of course you aren't immortal in this world, but I guess as long as you get into the good side of the goddesses, you'll keep on reincarnating

I'd imagine universes currently in war or medieval eras would be one of the more difficult ones due to the lack of powers or plot armor to save you.
gpt
I'd probably go with the Warhammer 40k universe for hardest. The casualty rate for "guys at the bottom" is so high you'd have a better chance of winning the lottery at the exact second you got struck by lightning than surviving, much less advancing all the way to the top.