Even vs AP team is OK. Rammus have more crown control and HP with frostfire Gauntlet. Frostfire Gauntlet is very strong to farm Jungle and it is strong to duel. Almost champion can't escape when rammus have Frostfire Gauntlet.

I am waiting for rammus mini rework to make best build for him

Nothing to change in season 10. Rammus now better Because have more 163 HP + 2s CDR E.
Focus 1 vs 1 and farm top lane. The minimum Rammus items: trinity force + Bramble vest + Ninja boots. Because with aftersock you will have more 400 armor.
When you can solo kill ADC or AP. The enemy can't win the game.
You can see here in my channel: In game I carry team to win even katarina have more 2 items. I have 440 armor with bramble vest + ninja boot + aftershock. In 2s, I can kill 1 and can get out.
https://youtu.be/On7w3AB5Fxw?t=1052

Beacause It give rammus CDR, Mana, HP, AS, AD, MS. All rammus need
Mana for Farm, use Combo and roaming (Rammus mana pool too loW). Combo Q,E,R,w =250 mana and you need 60 mana for Q to chase.
Attack Speed (AS) because of Rammus' ability to get 617 armor wihout items and by his passive Spiked Shell turn 15% of that into on-hit magic damage. That's easily 100-200 magic damage on-hit for the mid game to late game – during Aftershock these number even higher – so natually attack speed is not a wasted stat on Rammus.
HP: 250 is good and get rammus 20 MS when Hit enemy (60 Ms when kill creep).
CDR: With 20% CDR you will Use Q, W,E,R better.( 20% CDR Q: 6-->4,8; W: 6-->4,8s; E: 12 --> 9,6s; R 100 --> 80s)
Other side you will Push Fastter.
Combo Trinity + Thronmaill + Ninja tobi = Kill ad or AP