Best Ski Resort for a week in Andorra?

Soldeu is probably the best place to stay as the skiing subject to snow depths is not bad. However as you will have transport the best skiing in Andorra, particularly during the week rather than the weekend, is at Arcalis about 20 minutes from La Messana. Over the years I have had great fun skiing down some offpiste bowls and down to the La Coma Restaurant where there is a lift that takes you up to a great red (plus some offpiste black) when there is snow. Most resorts in Andorra are quite tame but Arcalis has some more challenging bits.
I assume you would drive up via Barca so I would advise not to even attempt to do the run on a Saturday when Barca goes skiing!
People can be a bit sniffy about Andorra but I have skied there many times and still enjoy it - when there is snow!