A protein skimmer pumps fine bubbles through dissolved organic compounds in your tank water. The bubbles attract proteins and other organics, which collect at the bubble surface and get carried into a collection cup as foam. The result is the removal of dissolved organics before they break down into ammonia, nitrite, and nitrate. A well-sized skimmer is the difference between a tank that requires daily intervention and a tank that runs hands-off for weeks at a time.
Manufacturer ratings are aspirational. A skimmer rated for 200 gallons is usually appropriate for a 100-gallon SPS-dominant tank, a 150-gallon mixed reef, or a 200-gallon FOWLR. The rule of thumb: pick a skimmer rated for 1.5 to 2x your actual water volume if you are running SPS or anything with a heavy bioload. A 100-gallon SPS tank needs a 200-gallon-rated skimmer. Undersized skimmers do not skim badly - they skim well at first and then collapse under bioload as the tank matures.
Royal Exclusiv Bubble King is the choice for serious SPS reefers. Built in Germany with cast acrylic bodies and Red Dragon DC pumps, they run quietly, produce fine wet/dry foam, and last 15+ years with minor maintenance. Pricey - the Mini 200 retails around $900 - but they hold their resale value better than any other reef equipment. Weakness: only the cost.
Reef Octopus produces the skimmers most reefers actually buy. The Classic series is reliable and affordable; the Regal series steps up to AquaTrance DC pumps and a quieter operation. A Regal 150-INT covers a 100-gallon SPS-dominant tank well at roughly half the Bubble King price. Weakness: pumps have a shorter service life than Red Dragons (5-7 years instead of 10+) and the foam fractionation is noticeably less fine than Bubble King.
Vertex Omega skimmers fit in tight sumps where larger skimmers will not. They run on Sicce Syncra pumps, which are reliable and quiet. The Omega 150 is appropriate for a 60-100 gallon mixed reef. Weakness: collection cup is small and fills fast on heavily-fed tanks; you may need to empty it every 2-3 days during the first months of operation.
Most reefers eventually move to a sump-based system, but the AquaC EV and Remora hang-on skimmers fill a real niche for nano tanks and apartment reefs where a sump is not an option. The EV-120 covers a 60-90 gallon nano with surprising effectiveness. Weakness: hang-on skimmers always have lower performance per dollar than equivalent sump skimmers; if you have the option of a sump, take it.