Chaotic rapier. Very powerful one-handed melee weapon that RuneScape gold you get after dungeoneering for some time: you need at least 80 dunge to buy it, using the dunge reward method. Most chaotic weapons are the most powerful in their own class. Bandos chestplate and tassets. Falling in cost, these are generally go-to items if you've got some money. 12M for a single piece, 12M for the other. They provide a power and prayer bonus.
I am not an expert on helms, however if you are slaying you would like the full slayer helm for its bonuses (likewise Salve amulet if it's relevant) and Verac's helm otherwise. For boots, you will probably want to stick to dragon. You will find better boots, called"ragefire" I presume, but I don't think you want to spend your bank on them. For gloves, possibly barrows gloves out of RFD or potentially goliath gloves in the dominion tower. Goliaths ask that you have done lots of quests and a lot of arcade-style combat in a big tower filled with pursuit bosses. Something you could pick up when you've caught up on your questing generally, I presume.
For the ring, there is a really pleasant free ring you get out of"A return to stone" (I think that is what the quest is called). It begins in TzHaar town, at the birthing pool. It is all of the fremmenik attack ring's bonuses, along with a lot of rather nice reusable teleports, along with a combat bonus contrary to the tzhaar. Or you might find the Berserker ring, which delivers a power bonus, or you could find the explorer's ring, which delivers a few free alchemy spells every day, or you could get an Onyx ring and also imbue it in mobilising armies.
Google that stuff if you want more detail. For your amulet, the fury (again, dropping in cost like bandos) is still generally considered the best bet. For the cape, you have the trimmed skillcape, or the flame cape, or the uber-fire cape which you get from giving a fire up cape and performing a minigame that's harder than combat caves. It's tough. Very tough. You have to do that quest for the ring I said previously. It will not look perfect, however from what I've heard it has great stats.
Asking for your ability which makes the most cash is extremely vague, and that means you're most likely to have (and have gotten) some unrealistic answers. I'll provide a run-down of a few skills you might be interested in (at the ways in which they're trained by the majority, therefore it is the most realistic - except for Hunter - jadinkos don't make money afaik but cheap OSRS gold they're quite fast.