While working in the lab with WA Dept. of Ecology's marine sediment monitoring team, I spend hours sorting through sediment samples under a microscope to sort/identify the invertebrates. We do a "broad sort" first into 4 phyla (Mollusca, Arthropoda, Echinodermata, and Polychaeta) and then these get further broken down into family and/or species level. There's a crazy amount of variety with the polychaetes ("tube dwelling" marine worms), and a good chunk of my time is spent disentangling them from one another for identification. I've become pretty good at immediately recognizing some polychaete species!
My favorites are always the little arthropods, however. In the marine world, these are the crabs, shrimps, ostracods, etc. The little hairy legs! The teeny black eyes! I die every time.