Every nadorc you run is a plain 750k profit.
Question: What's a nadorc?
True that diplomacy makes you run Tour faster and Tour of Universe gives 500K ec at least per day, which is a good method if you're able to catch it on time.
But two reasons I don't count them as best in his case is because the OP said he does not play often and therefore his options are limited to finding what he can at the moment.
And I disagree that ec is not a priority while leveling. You can do both at once and have ec in hand for when you do reach 50, because you'll need 20 mil - at least 5 mil for the survivor only package - to get important stuff you're dying to buy for your dream ship build, including consoles, weps, etc... .
a) you have to catch tour on time which makes it an unreliable source of income except for steady players.
b) diplomacy and di immunity are mostly good for Tour and that is as I said limited time only
c) doffing is an opportunity, but doing doff missions are unreliable way to base an income for the OP, as they need to be done steadily and in quantity to generate any substantial amount of profit. No mission I have encountered yet just gives 750k ec upon completion and I've done doff assignments in every sector of the map.
Few final thoughts for the OP:
Experiment, go through all the suggestions and find out the methods that work best for you. There is no shortage of ways to make ec on Star Trek. In fact, one of the many things I like about ST is the possibility is available to everyone to attain whatever goal in game you desire.
My suggestion for players leveling up: Don't upgrade your ships. Use the equipment it comes with, and sell every drop item you get on the exchange (if you can get more than 50% list price) or to a vendor ( if you can't). The exception being "common" quality items, as you can use them and still sell them later. Don't buy anything you don't have to, items get really expensive once you hit level 50 ( I just dropped 56m ec on two consoles) and you're going to need the cash. also, if you see an exchange item listed for less than 50% list , buy it and sell it to a vendor. You may only make a few hundred ec, but do it enough and it adds up.
Also , save your dilithium. Don't convert it to zen unless you reach your cap. And if you do convert it to zen, leave it alone. You'll need it to buy a Z-store ship at level 50 But don't buy Z-store ships while leveling up. Why buy a ship you're only going to use for 10 levels? I made that mistake. Now I have a bank full of useless universal consoles. But if you just have to have console XYZ, wait to you hit level 50, get into your endgame ship, and then decide if it's worth having.
Another alternative is to save your zen, and buy master keys to sell. Wait until you have enough to buy a 10-pack, as they're a little cheaper per key than buying individually.
I have other ways of making ec ( i usually can make 1m a day) but I'm not sharing everything. I don't want the competition.
Here is another easy way to make EC. Go to Kerrat, The borg drop good loot you can sell to vendors. I am level 50 and in about 45 min in Kerrat I can make about 500 to 600k in loot drops. You only need to watch your back for BOP's now and then. Also I made about 50 mil EC buying the Phased poloron wep packs and selling the XII Acc X2 weps from them for 5-15 mil a pop. The Wep packs cost about 200k on exchange. I invested 15 mil and walked away with about 50 mil. Im not sure if the missions are still there but there are some foundry missions that you can make about 300k to 500k in loot drops also.