Do you have livestock that needs to be herded? If so, get a Border Collie, as they are the premier herding dog.
You should consider a Border Collie even if you don't have livestock that needs to be herded, since a BC will certainly enjoy roaming the farm. Also, if you get a BC, you can teach it to play Frisbee. BC's excel at Frisbee, and will literally want to play every minute.
Otherwise you could get a Golden Retriever, they are great companion dogs. Also, a GR would love to go swimming if you happen to have a creek or lake on your property, a would a BC.