I would honestly say there's nobody rewarding skill to OSRS gold train. They all are helpful to some degree, and just training one won't assist you in the long run. That said, here are a few suggestions and the motives behind them: Construction. One you can have a portal room, a menagerie, and a prayer altar, this becomes extremely useful. The portal room is actually helpful, especially so when you are starting out as a part and most of the teleports you will use are not there. The POH is also perfect for storage, saving virtually every instrument, most novels, and a lot of armour and other clothes.
Agility can be also very helpful: getting it to 21 is essential so that you may use the Varrock-Edgeville shortcut, and obtaining it higher is very helpful in outrunning creatures, huge quest places, and even PKers (true experience there). Getting it to 21 should be one of the first things that you do.
Summoning is a great ability, but you can't do it instantly. If you haven't previously, unlock 43 prayer with penguin points and also the troll invasion DnD, and then put them into summoning. Being in a position to get a soul terrorbird (level 52) is very worthwhile and beneficial. What I am saying is. . I certainly would not call it the single most important thing: just don't ignore it. Learn how to farm things and level it up gradually. At around level 30/35 you'll have the ability to make some rather regular money from it.
Slayer isn't worth power-training yet. Do it if you feel like it. When you hit 55 it is useful as it unlocks a few handy new things, but before you get to buy RS gold the point where you want that, do not bother. It will make some money, but it is not actually a money-maker. Not in a minimal level.