Exalted Argent Champion of the Horde

Just to clarify...you will need to get exalted with The Argent Crusade, Darkspear Trolls, Undercity, Orgrimmar, Silvermoon City, and Thunder Bluff. So you will also pick up the Ambassador title along the way. On top of that, you need to spend over a month doing Argent Tournament dailies for each city in order to become a champion of that city, and if you are also exalted with that city, you get a title (<name> of <city name>, for each city). So all in all, if you plan to complete this achievement, you are looking at gaining six different titles on your way to Crusader, nice little bonus in my opinion.

25 days does not seem THAT bad, especially comparing to the cooking quest grind for the recipes etc...

Compare it to the 148 133 days^ of Champion's Seal dailies and 2500g' needed to buy all 5 10 ground mounts (Horde | Alliance, 100 seals each for new models, 5 seals and 500g' each for old models), and the flying Argent mount (Argent Hippogryph, 250 150 seals).

If you plan to buy all 5 of your faction's small pets (Horde | Alliance, 40 seals each) with your* Seals, that's an additional 40 days.
Buying all 5 of the Tabards (Horde | Alliance, 50 seals each) is an additional 50 days.
Buying all 5 of the Banners (Horde | Alliance, 15 seals each) is an additional 15 days.

That's a net total of 253 238 days^ of Champion's Seal dailies.
Buying each of the small pets from a different player saves you 40 days, for a net of 213 198 days^ of Champion's Seal dailies.

These numbers can be further reduced (however, slightly) by picking the Champion's Purse as the selectable reward for these dailies (instead of the Champion's Writ), because as of Patch 3.1.2, it has a very small chance of also containing a Champion's Seal.

*Note: each small pet is not BoP, thus allowing you to find these from other players.
^Note: the 10 Champion's Seals, or 2 days worth of dailies, that you get from A Champion Rises (the one-time quest given at Champion's rank) is deducted from this number.
'Note: this value is reduced by repuation discounts.

EDIT: Numbers/Info updated for Patch 3.1.2, in which 5 additional ground mounts were made available, the price of the Argent Hippogryph reduced, and additional Champion's Seals made available through the Champion's Purse.

To become a champion with a race not your own, you must first become a champion of your own race, and then you are able to repeat valiant quests for all of the other races, thus becoming champions with them after a certain amount of time.