The best...grind? How the hell could anyone have a favorite grind? Maybe it's just the Stockholme Syndome speaking, but I have had my fare share of favorite grinds. Kallixta suggests that "grind" refers to reputation, but I think that we do many grinds in game - leveling, professions, and gear, in addition to rep. Some are more voluntary than others. Even hardcore achievement chasers may love the final confetti and badge but hate the grind to get there.
For instance, I didn't particularly enjoy the grind to The Insane (what? really?). I did, however, enjoy grinding Brood of Nozdormu rep as part of the Scepter of the Shifting Sands questline. Although the quest chain had many grindy elements, the varied and often humorous quests made it worth it. The rep and the quests could be thought of as two separate grinds, but honestly I enjoyed them both. Of course, the rep consisted mostly of staring at my furry bear butt mangle-spamming those Anubis dudes, but there's a certain peace to be found in that. Somewhere.
I also enjoyed the lengthy grind towards tier.5, most of which was grinding for the damn gear before you could even do a quest! UBRS, I never want to see you again :) That I think I enjoyed more for the final quest chain than the gear-grind beforehand. Low level instancing gets on my nerves because the mobs don't pat and I have to force myself not to loot since my bags are always full anyway. (But I love looting ;~;) The final quests were really interesting, though, and I love the look of my new gear!
Netherwing was also enjoyable for me. I liked the progressing story and found the orc peons hilarious. Hunting for eggs never worked for me, though. I just did all of my daily peon-booting and flayer-murtilating for ages and ages, along with the totally nerfed flight race, and they gave me a dragon friend!
What these grinds have in common, looking at them, is that they have a story and I did them above-level. However, I also enjoyed grinding out all the Wintergrasp achieves. I have no idea what WG's story is, and the quests were only good for shards, but the battle was fun. I think that caring so little about PVP makes it easier for me to enjoy battlegrounds, even when I'm getting killed all the time.
What does "grind" mean to you? Do you have any favorites, and why?