Once you have the rows in an SPListItemCollection, you can do a count on the rows and get the ID from the last one, putting it into a variable, like this (I use a string variable, for simplicity with casting):
string myCAMLquery = @"
<View> <Query> <OrderBy> <FieldRef Name='Created' Ascending='False' /> </OrderBy> </Query> </View> ";
SPListItemCollection myListColl = myList.GetItems(myCAMLquery);
int count = myListColl.Count-1; // because it starts at 0
string myID = myListColl[count]["ID"].ToString();
should work.