What mutation is the best to get?
The answer here is as individual as there are people. There is no perfect bird or mutation, and you as the purchaser select the mutation or species / sub-species that most appeals to you, fits your own needs and expectations and is something you admire. Most individuals can afford the more common varieties or mutations, but they start to increase quickly in price and restricted availability as you seek out new mutations. Several years will pass before you ever obtain an opportunity to buy a relatively established mutation at a reasonable cost (or at least at a cost that reflects other mutation values on the market).