No. 1: Ketel Marte
This might be cheating as the likelihood that Arizona decides the trade the best second baseman in the game is slim to none. However, if the Diamondbacks are looking to clear some payroll while rebuilding for the next few years, they could entertain offers if they are blown away by a deal; the Mariners are one of the few teams that has the firepower to make a blockbuster like this happen.
Marte fits this team to a T. The 30-year-old is the best second baseman in the game, hits for a high average, limits strikeouts and has really developed a lot of power. The switch-hitter is also a fast runner, but not necessarily a base steal, but who really cares when you hit as well as he does? He may be in the midst of his best season yet, slashing .283/.351/.506 with 18 home runs. Thats all good for a 136 wRC+ (tied for first among all qualified second basemen.
I doubt that Marte is really available as he is one of the best players in the game and under a very team-friendly deal, but one can dream, right? Acquiring Marte would be extremely expensive, but might be worth the cost. You would likely be parting with 3-4 top 10 prospects (maybe even more) or one of Bryce Miller or Bryan Woo and a few more top prospects.