One of my favorite things to do in Dominion is playing as "Masochistic Rammus." Basically what I do is simple: get all the items that punish people for being around me for too long/ build up survivability.
My build is generally always the same with some rare exceptions. I build things in the same order (most of the time) as well. It goes as follows:
-Boots of mobility
-Force of Nautre
Why I build him this way:
Even if the enemy team is all ap (obviously a bad choice) I still do a lot of damage against them throughout the entire game. Early game is the hardest against a more ap-centric team as Rammus, but getting to the top really fast as well as having extreme mobility helps give my team a head start to capping windmill.
I get all my skills first, then max out my defensive ball curl as soon as I can (prioritizing my ult of course). This helps me maximize my damage output and defensive capabilities. My second skill I focus on is Taunt, the longer it last the better obviously, and I don't level up powerball because no matter its level, it never makes you go any faster.
I build sunfire cap because of the extra magic damage it does and it gives me survivability. The main goal of this build is to get people to kill themselves, with as little damage to you as possible. With that in mind, having extra health is never a bad thing as it makes you harder to burst down and can help you out-live those hard hitting ad champs.
I get FoN for some obvious reasons. Firstly is the movement speed buff, not significant at 8% but every little bit helps. Secondly it stops those ap champs from annoying the Hell out of me, as that's all they can do once I get a giants belt. Once you add a FoN, they're all but useless. Third, the hp regen is a nice little bonus for Rammus, and even with 20% healing gone I notice a very big difference.
The build is a little expensive, but the first items are really cheap for the stats they give you. Boots of mobility help you either stay with your team so you can tank, or give you the ability to back-cap. Thornmail is great with your defensive ball curl and your passive, so not only does this give you armor, it gives you attack damage and an awesome unique passive. Sunfire cape helps deal even more damage and gives you survivability, and FoN just helps with general survivability. The most expensive items are FoN and sunfire cape, but if you can afford it it's worth it.
Now some people would say he still has counters, but even on a team of singed, ahri, and morg, I have 1v3'd them and still came out alive. This Rammus is by no means unstoppable, but he doesn't have to be. All he has to do, is piss off the enemy team enough to the extent that they chase him, focus him, or just get distracted by him. Even if you die, they are leaving points undefended and teammates unharmed.
Some would say,"oh, just buy some magic resist and it will counter your thornmail and sunfire cape," well here's a news flash, it doesn't. It just gimps your damage and lets my ad chip away at you until you're either dead, or too low on health to stay in the fight.
The best way to counter this Rammus is really to kill all of his friends instead of focusing him, and then turning your attention to him. Rammus is a great tank with a lot of reason to attack him (meaning not easily ignored) but if you can live through his ult and the magic damage, or if you can outpoke him some how and use chain cc, you may be able to kill him. However, fighting him 1v1 off of a point is highly unadvisable.
Now, some other Rammus players may wonder, why not Omen or Frozen heart? Well, if you want people to attack you the last thing you want to do is slow them down. Both of these are GREAT items that can help your team survive, potentially, and they even help your passive, but this Rammus greatest strength is in his own pain.
If you think a normal Rammus blows up a Xin or a Master Yi, just look at this one. There is almost nothing as satisfying as watching someone kill themselves in the fastest way possible.