Most and least useful bit of equipment

Most - Wet and dry vacuum cleaner - it gets used with almost every job.

Least - Satellite TV antenna/receiver - I never get the time.
 
Most useful, for cruising Bahamas and the Caribbean:

Davits, so that we could handle a decent sized inflatable dinghy and motor.
Shade - in our case a variety of canvas squares with grommets so that they could be deployed as necessary depending on where the sun was. It's particularly important to be able to deploy a sunshade vertically at the aft end of the cockpit, because the boat lies head to the prevailing easterly wind and the sun goes down in the west.
Canvas bucket with lanyard.
Fridge.
Good permanent boarding ladder - an essential safety item as well as a convenience.
Jerry cans for water.
Jerry cans for fuel, with effective pouring spouts.
Twin high output alternators.

Least useful:

Forward looking sonar.
Wind generator.
Water maker.

Most wanted: A 12v pop-up toaster!
 
Loads of good advice already so I'll forget about biminis and the like. However following on from SaltyJohns toaster wish.......get a ridged, square griddle pan. On the stove top we make brilliant toast in ours. It had been at the back of a cupboard at home for years so we took it to the boat. Inspirational !!! Do not bother with those ludicrous toaster efforts they sell in chandleries, waste of money.
A few solar powered garden lights never go amiss either. We got 10 for £10 in a sale at B&Q. Used day in day out to guide us back to our boat at night. Also double up as emergency anchor light.
 
Not necessarily the MOST useful piece of kit, but certainly time/water/stress saver.... large travel towels like these http://www.lifeventure.co.uk/htm/washgear/soft-fibre-trek-towels.html

They're very easily washed and rinsed on board and dry in no time at all, unlike normal terry towelling towels, which use up huge amounts of water when hand washed and take forever to dry. Wouldn't be without them even though they're a rather pricey initial investment.
 
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