As mentioned above, feather duster/fan worms seem to be the easiest to care for, then things like barnacles and a number of different bivalves (clams, mussels, oysters, etc. - some bivalves, such as flame scallops, seem to have very specific feeding needs, so I'd consider them more difficult); some people have had some success with NPS corals; sponges and certain specific NPS corals (like the chili corals mentioned above) seem to be the most difficult to keep long term, and I only know a couple of people who have had really any success with them at all.
For the easier care specimens like feather dusters and bivalves, you typically just need to position the specimen properly (meaning in an area with the right substrate, the right flow, sometimes the right lighting, etc.) and dose phytoplankton (from what I've seen, I'd recommend Isochrysis galbana [T-Iso], Chaetoceros sp. [and/or Thalassiosira sp.], and Tetraselmis sp. - a blend containing all of these with or without other phyto species would be ideal) in high enough quantities for them thrive. (I'd also put things like bryozoans in this category.)
For slightly more difficult specimens like barnacles and easier NPS corals, you need to position the specimen properly and feed meaty foods/pods (pods referring to copepods, rotifers, ostracods, amphipods, brine shrimp, etc.; basically any small, feeder crustacean) in high enough quantities for them thrive.
For difficult NPS corals, you need to position the specimen properly and feed some kind of meaty foods/pods (nobody seems to know exactly what meaty foods or pods though) in high enough quantities for them thrive. (I'd put NPS hydrocorals slightly below this category, but above the barnacles category at this point.)
For sponges, you need to position the specimen properly and feed the right kinds of foods in high enough quantities for them to survive (I don't know if I've heard of any non-invasive sponges actually thriving in a tank yet) - the main problem seems to be providing the right kinds of foods in sufficient quantities.
Long story short, the only "success" stories I've seen with sponges either come from labs (one of which was able to keep the sponges alive and healthy enough to reaggregate while also being somewhat unhealthy and clearly lacking in color by feeding a bunch of phyto while the sponges likely also fed on bacteria, cyano, and possibly diatoms in the tank; the other managed to keep a known invasive species of sponge alive in a similarly mildly healthy/unhealthy state, but I don't remember the methodology off the top of my head) or from people regularly stirring their sandbeds and/or blowing detritus off their rocks - this is thought to help get the bacteria, microalgae (phytoplankton and similar), and appropriately sized organic matter pieces from the sandbed/rocks into the water column for the sponges to feed on (these seem to have similar levels of success as the labs mentioned above, with periods of both sponge growth and sponge deterioration).
