what plants are best for a west corner very shaded

I recommend ligularia skyrocket, yellow flowers and awsome leaves.

Sorry that should have read ligularia the rocket.

How about a Japanese acer Acer Palmatum Altropuruem
They love a nice shady spot as long as its not to windy and are gorgeous deep red foliage

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If you want a shrub that will fill quite a lot of it up, Mahonia aquifolium - evergreen, gets about 3/4 feet high and the same wide, yellow mimosa like flowers in early Spring. If its damp as well as shady (not if its under a tree ,for instance) low growing plants are Brunnera, Ajuga reptans, Pulmonaria. Foxglove, but obviously this is much taller! P.S. You might find coloured leaved acers (mentioned above in one of the answers) may not get enough sun to keep its colour if its really shady)

Another super plant if we are talking smaller plants:

Ophiopogon planiscapus Black mondo grass, will make a nice 18-inch-wide mat in three years and is actually of the Lily family.

Go for a rose like an Iceberg (white) or Teasing Georgia (yellow)

http://www.caringforroses.co.uk/page_1247170078066.html