Starting out as MIN/BOT shards would net you the most money, but they're only level 1 so you can't actually get experience from mining/ harvesting them for long. Crystals are level 30, and clusters are level 50 unspoiled nodes (single use respawning for 9 minutes every 72 minutes -- 3 hours per Eorzean day). There are lots of things to mine or harvest that sell reasonably well. You'd have to change what you're gathering from time to time to actually continue leveling. Once you hit 50, you can gather darksteel ore, gold ore, clusters, spruce logs, and crawler cocoons for decent money. Unfortunately none of the items that they've added since launch are useful or sell well.