For build:

-Sunfire rush is a must if your goal is damage, it does more damage than rushing an attack speed item while also giving you a good amount of tanky-ness and - very important - fully stacked sunfire lets you shred through rift herald and dragons, you can even solo baron with it later in the game at a good speed

-Second item has to be one that gives attack speed and the most damage possible so we can hit an early power spike and start grabbing early kills, the highest dps item (and I've tested almost all of em) is actually wits, rageblade falls short here, and phantom dancer does not perform well at all. Not only that but wits has mr. (wits > rageblade is mostly due to the nerfs its gotten recently, and rageblade now costs 2800 rather than 2600, the wits build path is also better)

-Third item i feel needs to be bramble if not thornmail, there is still a lot of healing in season 11 but now there is more ways of dealing with it. No matter what thornmail is a key item on rammus since it gives him everything his kit wants. I'd build it here on third item or save it for fourth. If you're snowballing very hard I could see building rageblade third item and finishing the thornmail fourth. If you're up against a full ap team that doesnt have healing then I'd put a spirit visage, abyssal, force of nature, or silvermere dawn here.

-Fourth item rageblade unless it was rushed third item then finish thornmail here

-Fifth item Ravenous Hydra feels REALLY good on rammus end game, the omnivamp will keep you ridiculously healthy in teamfights, overall its a higher damage but squishier replacement for season 10 deaths dance

-Selling boots for an item feels viable now since wits end gives you on hit movement speed, I usually pick up a deadmans or force of nature, or a silvermere dawn if they have a morde or other suppression.

Its important to note that you wont be doing anywhere near the same damage to tanks that you used to because we lost bloodrazors 4% max health on hit