like most other people are saying... panasonic is the best for expandables...
on a side note, panasonic's current line of phones has many models in 5.8ghz that come with 1,2, or 3 handsets... be careful, some of them only work with the handsets that come with it; while other models (like the KX-TG6500 mentioned earlier) work with up to 8 total handsets that are sold separately. so don't buy one that has multiple handsets but is not expandable unless you're sure that the 2 or 3 handsets is all you're going to need.
btw: i sell phones and small appliances at an appliance store, and if you want to pm me, i could get you the model numbers of the phones i'm talking about that are not expandable