I voted Slavelords of Cydonia.
My rationale is that this is the purchase who will give you the most with regard of usefulness /time-saved for your money.
Well, three of these books I am interested in, the fourth (Sharn) not; so I am biased of course. Also, I assumed you intend to DM / run a game, not just be a player and buy for reading sake...
-- Complete Adventurer: (PC crunch) Seems to be an excellent supplement. However, it will be probably useful only a couple of times, mainly for one player character and a few feats. You could use it to design NPCs, but it will take time, and players won't really see the difference between a true ninja NPC, and a pseudo ninja NPC made with rogue + assassin.
-- Sharn: City of Towers: (city setting) Probably a cool supplement if you run Eberron. However, it's useful only for background info, and thus requires of you to have Eberron book, and then either buy more stuff or create adventures yourself.
-- Slavelords of Cydonia: (adventure + small setting book) You only need to have Grim Tales. Then, you have a module spanning an entire 1st to 20th level campaign. As such it saves you a lot of work, and doesn't require to buy anything else to immediately run a campaign. BUT you and your players will have to like this kind of different setting and d20 rules.
-- War of the Lance: (setting book) Probably an interesting read if you run a Dragonlance campaign. However, same comment as for Eberron: you need the Dragonlance sourcebook as well, and still must buy some adventures or write them yourself.