This is my first guide where I write chapters so it might look awful but bare with me here. Rammus was my first champion that I picked up and he has a very special place in my heart. I believe rammus is a great pick, as you can build tank and absorb damage or more bruiser style, maintaining your tankiness and one-shotting adcs.

Rammus top, unlike rammus jg, is a very situational pick in my opinion. The only situation where you can pick him is when:
a) you are VS a ad top and jg
b) you are VS a yasuo
c) you are vs a yi
d) you are vs a heavy ad team.
Not only does he benefit from ad opponents since his w can stack a lot of armor, but also because his passive: the more armor you have the more damage on-hit you deal. So expect some occasional 1shots vs things like yi and yasuo.

A) against a ap top
B) against a ap heavy team
C) against vayne
Rammus suffers against ap teams like any other tank(except ornn and mundo) since there arent many good magic resist items in game suitable for him. Also vayne, if enemy picks vayne there are 2 things you can do depending on situation:
1) If you havent picked yet, simple dont pick rammus
2) if you have picked rammus just dodge it is very unlikely that you will be able to do your job against a vayne since she cancels your q and makes your w useless.

This is it. I hope you found it useful. Thanks for reading. Make sure to read my other guides too!