The best single piece of equipment that you can have is literally right in front of you. It sounds lame, but seriously, this board contains EVERY single bit of information that you could ever need, and if you cant find it, just ask!
What i would suggest is that you search around here, ask questions, and read a good book or two . . . THEN get started. Trust me, although it can be difficult to be patient and wait, you will save yourself some money and some frustrations.
BUT To answer your question:
List of essentials:
1. Tank (duh)
2. Water (best bet is reverse osmosis)
3. Synthetic Sea Salt (instant ocean is a quality product)
4. Substrate (stuff on the bottom) this is a currently debated topic in the reef forum
5. Live Rock for filtration
6. Powerhead for water circulation
7. Heater
8. Way to measure salinity
9. Thermometer
List of optionals:
1. Protein Skimmer (removes dissolved organic compounds)
2. Refugium
3. Sump
4. High Powered lights (I say that lights are optional, when they are actually necessary to see your tank, but if you plan to keep corals you must have high powered lighting. This is THE MOST expensive piece of equipment)
There are definitely other pieces of equipment that are needed in maintaing a saltwater tank, but these will get you started on the right track.
HTH
Justin