I gave red chinchompas as the profit earning alternative.

For your ring, there's  OSRS gold a very pleasant free ring you get out of"A return to stone" (I believe that is what the quest is called). It starts in TzHaar town, in the pool. It's all the fremmenik attack ring bonuses, along with a lot of rather nice reusable teleports, along with a combat bonus against the tzhaar. Or you might find the Berserker ring, which delivers a power bonus, or you might get the explorer's ring, which offers some free alchemy spells each day, or you could get an Onyx ring and imbue it in mobilising armies.

Google that stuff if you want more detail. For the amulet, the fury (again, dropping in price like bandos) remains generally considered the best choice. For your cape, you've got the trimmed skillcape, or the fire cape, or even the uber-fire cape that you get out of giving up a fire cape and doing a minigame that is harder than fight caves. It's tough. Very tough. You must do that quest for the ring I said previously. It doesn't look perfect, but from what I've heard it has great stats.

Asking for the ability that makes the most money is quite vague, and that means you're likely to have (and have gotten) some unrealistic answers. I'll provide a run of a few skills you might be interested in (in the manners in which they're trained by most, so it's the most realistic - except for Hunter - jadinkos do not earn money afaik but they are quite fast.

I gave red chinchompas as the profit earning alternative. Grenwalls are another option and more likely to make money but they are a lot more concentration intensive and also a lot slower exp/h - like runite's relation with Mining). Fishing out of 90-99 would make about 46M on Rocktails (I didn't include the purchase price of living minerals, but I am making an assumption that will be negated by any gains made getting to 90 Fishing), carrying about 180-190 hours (again excluding  buy RS gold levels before 90, and assuming ~42k exp/h).