The humble weed 'Pulsating Xenia', I did a review of cultured corals we have sold over the past 10 years.
Xenia may be the cheapest but prolific I have sold many thousands of them. It always surprises more advanced reefers just how high the demand is for them.
I'd have to say my Red Spotted Mushrooms. I've traded more from equipment, corals, salt and additives than a 10th spent out of pocket for anything I have. Easy to maintain and fast growing to say the least. I catch them sometimes floating in the current because there was no room for them to walk. Gold Jerry, Gold!!!!
Xenia is a huge seller. For every SPS dedicated guy there are many more people who can't keep stonies let alone SPS. And to people starting out a really cool bright xenia with lots of movement and color that is also easy to keep is about as good as it gets.