I would actually say make P1s and P2s, rather than P0s, as there’s usually someone looking for those. Many P1 and P2 commodities are components to things made in Industry, so industrialists buy them up pretty reliably. P0s you’ll have to sell for chump change, and even still, you have to hope that someone actually needs them. Supply/demand.
Easiest money depends on where you’re selling it. For example, I work out of Molden Heath, and I can usually sell P2s like Mechanical Parts there for good isk, but other items like Transmitters and Livestock (ie without an immediate use) have no buyers. I have an order of Transmitters on the market that I’m no longer expecting anyone to buy. I’ve had much better luck with selling Transmitters at Rens.
(If you don’t have a particular market in mind, try using the Eve-Marketdata search function to see where stuff is selling the best.)
If I had to absolutely point to one commodity that always makes isk, I would say Nanite Repair Paste. PVPers always need it. But it’s an involved process that requires multiple planets and an Industry blueprint, so that’s more of a thing to work toward rather than an immediate moneymaker.