If you were to decide about which Dlc is a must\has to be played AT LEAST once, what would you answer?

Shivering Isles. Lots of new locations to explore.

Agree.
You get a lot of new items, quests, enemies, etc... And it's one of the best TES expansions in general.

All the other DLC for Oblivion is made of small additions with hardly anything to actually do.
Except Knights of the Nine, although that quest is quite short too and you only get a handful of new items.

I agree with this as well.

Of the shorter DLC, I will say that I enjoyed the quest for Mehrunes Razor. A lengthy dungeon crawl, with nice rewards. Still, Shivering Isles is the favorite for me.

Shivering Isles is an absolute must. Best DLC in a game ever.

For me, SI is essential because so many mods I use are dependant on it, though I rarely go there. Like others have said, the rest are pretty optional. I purposely don't have KOTN because I don't like what it does to the Anvil Chapel, so I would only add it if I were going to play some type of a knight or cleric.

The other small DLC I like is spell tomes, but I find Maskar's Overhaul to include a similar feature, so I don't bother with Spell Tomes in my PC game. On Xbox I have all the DLC but on PC its just SI.

KOTN is good for what it is, so it should be played at least once, but it is not much compared to SI. The rest just add a little flavor and are not in the Must Play At Least Once category, at least not for me.

I make a distinction between DLC and expansion packs. Shivering isles is an expansion in my book, not a DLC. http://www.uesp.net/wiki/Oblivion:Official_Add-Onsaccording to the UESP.

I treat DLC the way I would any other mod. I install it if a particular game and a particular character requires it. But I suppose I'm most likely to use Wizard's Tower with the widest variety of characters. The tower, in my opinion, is one of the few examples of genuinely creative, imaginative architecture to be found in the game. I love seeing it off in the distance, even if a current character has no plans to ever go there.

It is a cool tower. If it were not for the seal on my KOTN disc, I might be tempted to drop it into my game. But I played so long with the tower on my Xbox that I don't miss it too much in my PC game.

EDIT: Since DLC stands for downloadable content, which refers to the way the product is distributed, rather than what it contains, I am not sure the distinction between DLC and expansion is meaningful. Both KOTN and SI are (or were) available both on disc form and through internet download on all three platforms.

Have to go with the Majority Vote.

Shivering Isles.

I've yet to play a piece of DLC that gave me even half the enjoyment this one did. Of ANY game.

To be honest, I only experienced Shivering Isles and Knights, and between those two SI wins without a single thought given into it. And frankly, since I know what other DLCs add, I'm not sure how could they be compared with a whole bunch of new land, new monsters, new items, a whole main questline (and more) for me.

If we're including the expansion, then I'd say Shivering Isles too. But not for the reasons most people cite. I keep it installed solely because so many of my mods require it. It's my least favorite Bethesda expansion. I've tried to play it a few times and have never gotten farther than Xediliian.

Obviously the horse armor is the best DLC ever.