Ah, that's going to be difficult!
returns 1/2 hour later with 8 tankos
Well, I've narrowed it down to 8 – so that's a start.
walks off, then returns 1/2 hour later with 4 tankos
Ok, I've got it down to four now, and my final four actually split evenly into two groups of two side-by-side volumes. I guess I'm just a freak for good transitions or something. ;) Anyways ...
Volume 10 is everything everyone's already said plus : in 10 we see Sanji figuring out his place among the Mugiwara and the beginnings of the ongoing Alpha battle. We see Usopp recognizing his weaknesses and winning a battle in spite of them. And Zoro's fever chills and fainting spells add the right amount of suspense/tension to an otherwise silly (but highly entertaining) battle with Hachi. We can't be certain that he'll win it, even though he just promised Luffy that he'd never lose. And here we see Nami putting her faith in Luffy entirely. Art-wise, tis volume contains one of my all-time favorite pictures : that wild deep-perspective pic of Luffy falling backwards out of the sky shouting "I'm BBBAAAACCCKK!"
Volume 11 wraps up Arlong Arc tearfully but joyously and introduces us to Smoker and Tashigi, as well as Yubashiri and Kitetsu. Mihawk and Shanks are shown to be on speaking terms (small world, eh?) and Buggy makes yet another FLASHY appearance. It's one of those transitional times where humor carries the day and where so much of what will become important down the Line is hinted at or introduced without much fanfare here. Oh, and Dragon too. ;)
Everything everyone else said, plus : Some of the most cinematic art of the series as well as a satisfying wrapping-up and unification of all the seperate strands of this arc, including the Marines, the Rebels, the Royal Family, the Mugiwara, the remaining Baroque Works agents, etc ... speaking of which, 2-Bon's final act of kindness towards the Mugiwara, and his many quoteable speeches ("can you eat your breakfast tomorrow knowing that you have abandoned your friends?" and "One may stray from the path of a man, etc ..." ) are yet more reasons to love this volume. And if you needed additional stuff, there's also a bit of not-too-embarrassing fanservice (the males and the females) in the Royal Bath section. There's also the visual joke that so many people miss (probably because it's so small and only one frame long), where Eyelash, the perverted camel, somehow sneaks into the bath and takes Zoro's place while the Mugiwara are spying on Nami and Vivi.
Volume 24 gives us one last sweeping cinematic look at Alabasta, then shows us some interesting developments between Smoker and Tashigi (he's making coffee for her for a change!) as well as giving us the biggest shock of the series when Robin suddenly appears.Then there's the mysterious ship from the sky, the looming shadows of Skypiea, the introduction of Blackbeard's crew, the promise not to fight (and the fallout of that), and the annoyingly fashionista crew of Bellamy (who we really, really really want to see get what's coming to them -- though that won't happen for a while yet), the silly antics of Shoujo and Masira, etc etc ...
The also-rans were 33 (DBF), 39 (Sanji, Soge and Franky on the train), 25 (Southfacing bird and the larger political situation in the Oda-verse), 21 (three entertaining one-on-one fights w/BW)