When you are talking about slim, modern day tablets, the iPad was the first to market. Before Apple's slate, the "tablet" category was filled with clunky devices that ran desktop software and carried price tags over $1,000.
Obviously, being first has paid off for the company in dividends. I don't think anyone would argue that the iPad has dominated the tablet market over the last two years, and it doesn't seem to be slowing down anytime soon...