This is such a huge question as the properties available are so many, and so varied. If you have an idea of what it is you want out of your villa then you will have a good starting point. For example we always specify the following:
South or South/West facing pool
Pool area not overlooked by anyone
Games room - including pool table
PC with interent and Wi-Fi (free of course)
Free international phone calls
Single story villa normally, but if it is a double at least one of the rooms must be downstairs.
We have always rented villas from the owners on the site below
http://www.orlandovillas.com/
and have had no problems at all so far. Most of the owners are based in the UK and are very good at answering any questions, and responding to problems.
As for the best areas to stay at, this is another question that will get you lots of different answers as everyone has their favourites for a variety of reasons. The first few times we went we stayed on H27, this time however we stayed on Ponciana Blvd and thought it was a much better location. Closer to the parks, but also closer to Old Town - which we visit several times per trip.
Hope this helps - but if not feel free to ask some more.