Some of the country's leading stores are rewarding their best customers with exclusive offers, discounts on credit and bank cards, easy returns and special "welcome" packages. Macy's, a unit of Federated Department Stores, pampers its best or most-frequent customers by offering five "clubs," with successively better deals as the customer spends more. "Rewarding the best customers is growing and moving now into the upscale realm of retailing," said Arnold Aronson, managing director for retail strategies at Atlanta retail consultants Kurt Salmon Associates. Rewards are also invading the hotel field. Hilton Hotels is offering the 5 million travelers who have become members of its Hilton HHonors club "double points" or credits in its hotels and extra miles at airlines. Holiday Inn recently ran magazine ads entitling guests who presented coupons clipped from the ads to an extra night at one of its hotel-motels.