This is three different tank rammus builds. The first one is for facing a team that is fairly balanced. The second is for a heavy ad team and the third is for a heavy ap team. For the balanced build the first five items I highly recommend. The sixth item is your choice depending on whether you want more armor, health, or magic resist. If somebody on the other team is fed then I would recommend you get the last item to help defend against that. If your team is doing good and you sustain really well in team fights then you don't need anymore tanky items. So your last item can possibly be a rod of ages or anything else that gives you ability power since most of rammus's abilities do magic damage. You don't have to get ability power if you don't want to but if you want to do more damage I recommend getting some ability power.

For this build I suggest getting all the items in the build because if you're going against heavy ad team and some of the enemies are fed, you're going to need all the armor you can get if you want to sustain in the team fights. The guardian angel is to give you more armor and a little bit of magic resist to counter against anybody that may do magic damage. For an ap team it's the same concept. Just get lots of magic resist and once again the guardian angel is to give you a little bit of armor for any physical damage you might take.