Hi John,
There are as many answers to this as there are days in the year lol!
It depends on a few factors too
Position and elevation of the site - how high above sea level are you ? what direction does the area face ? exposed and windy or sheltered and frost pocket ? does the area get full/partial shade ?
Soil type and condition - just what it says, some plants do well in acidic soil, some will shrivel and die. You can either do a soil test or see what's growing roundabout - if you've lots of rhoddies growing there's a fairly good chance your soils acid. Is it heavy and clay or light and sandy - again tailor the plants to the soil type (or import lots of new sandy loam for optimum growing conditions) or else you'll find they don't give you the best display.
Lastly - what are you looking for ? Do you want low mainenance, chuck 'em in and let them get on with it or are you up for a bit of cultivation ? Also in terms of colourscheme do you want a riot of colour with mixes of red, yellow, blue, orange etc or a more subtle sheme with one or two colours.
A few suggestions straight off for that farmhouse/cottage garden look are,
foxgloves (but they are poisonous)
ivy (to climb up and along your wall, great backdrop for setting off plants and a haven for birds and bugs)
geraniums
carnations (dianthus)
clematis (again, good climber/rambler - train it along the wall or over a gateway/arch)
daisies - ox eye, michelmas - the list is endless
Echinacia (ooo, sorry about the spelling !)
Buddlia (good, taller plant, great for the beasties )
Oh, and go nuts with bulbs ! Daffs, snowdrops, allium, tulips - get different varieties that will cover you from spring right through til late summer, they'll just keep popping up year on year
Don't forget to include some taller, evergreen shrubs too, for year round intrest and the wildlife in your garden !
HTH
Karen x