Are the major DLC additions for Oblivion (namely Shivering Isles and Knights of the Nine) better to do before or after finishing the main quest?

To give some context: I'm still on my first playthrough (that lasted more than a couple hours, anyway). I have never finished the main quest – at first because I erroneously thought that would end the game, Fallout 3/New Vegas style, but eventually just because I got the impression that my character is the sort of fellow to shirk duty in favor of just being recognized by everyone in Cyrodiil as totally awesome. So, instead, I've explored a ton, done nearly all the Daedra Shrine quests, finished every guild/arena quest, and sort of accidentally did just enough of the main quest that I spend a lot of my time dealing with random Daedra jerks and collecting Sigil Stones.

I got the GOTY edition, though, so I'm trying to decide what to do next. (I searched to see whether there were any other threads on this, but the most I found was that people said they prefer to explore before doing the main quest.) Are Shivering Isles and Knights of the Nine better to put off until after the MQ, or better to get to earlier?

I guess implicit in my question (without welcoming spoilers, I hope) is which quest is the best to END the game on. I assumed it would be the main quest, which is another reason I'm putting it off, but if one of the DLC quests has a truly climactic ending, that would be good to leave until later. I know a lot of folks around here have been playing for five years and counting, but my girlfriend has made it very clear that if I don't move on to another game after finishing all the major quests, I am a dead man. (I have to at least go beat another game or two before I'm allowed to come back to start my vampire roleplaying playthrough...) So, please feel free to let me know if any one of these quests is best saved for absolute last.

Thanks in advance for your input.

Roughly, without any spoilers, I would tend to do it:

KotN -> Main Quest -> Shivering Isles

Well... a few not-quite-spoilers.....

KotN has some nice armor and weapons in it. Those items are leveled, but they're among the only items in the game with which you can improve their levels as you go up. With most everything else, they're fixed at whatever level they're at when you get them, but with the stuff in KotN, there's an almost automatic way to improve them as you go along. So it's pretty much the only quest in the game in which you don't effectively get punished for finishing it before the leveled items max out. And if you're playing a particular type of character (relatively good), those items can really help in the MQ. The downside to the KotN items though is that your character has to stay relatively good, or he can't use them at all.

The best things out of the MQ, as you've obviously already discovered, are sigil stones. They max out at level 17, so there's no reason to do it any later than that, really. The opponents get tougher and you get tougher, so it can be more entertaining to do it later, but there's no greater reward for doing it later.

The items in SI max out much later than that, so it's the best one to put off the longest. It's also the toughest, so you want to be as strong as possible. And the ending is truly epic, in every possible sense of the word. Honestly a much bigger deal, in my opinion, than the ending of the MQ.

I don't think there's anything too spoiler'ish in there. I hope not......

I don't think so either. Thanks!

I'm currently level 35. I'm finding most leveled stuff I get is maxed out already (when I compare what I got with its listing on the UESP wiki), but end-of-quest rewards won't matter as much for me after my "last quest" anyway. If Shivering Isles has a more interesting ending, that'll probably be my biggest motivator to save it for last.

Oh yeah - if you're level 35, none of that matters anyway. Everything maxes out (if I remember correctly) by level 30.

But yeah - even with that, I'd tend to save SI for last. You could do KotN or the MQ in either order though. Depending on how you feel about it, you might want to get the MQ out of the way now, just so you don't have to deal with any more Oblivion gates, since you do a lot of traveling for KotN, but that's about the only other issue I can think of.

You might want to go into SI and poke around a bit anyway though. It's a fairly long series of quests, and there are a lot of side quests too, and some nice stuff you can pick up along the way before you even get to the end. And you'll be able to see it coming when it starts getting really serious, so there's no real danger of finishing it before you intended to. And you can leave at any time and come back later without it making any real difference.

gpstr has already done a good job explaining, but I'll just give you my take as well. Given your level, you're not concerned with items, so I'll go just based on how much fun each quest is. I wasn't overly impressed with the MQ to be honest, and I didn't really like the ending. I think I've only finished the whole thing twice, unlike the guild quest lines, which I have gone through countless times across many characters. KotN imo, should be considered a side quest more than an expansion. It's not very long, and you basically know the story before you even go through it. I wouldn't recommend it very highly, It consists almost entirely of gathering the pieces of armor, which is pretty boring. Shivering Isles on the other hand, is amazing. It has an amoutn of content that rivals full releases. I personally enjoyed its campaign more than the MQ. If you want to save the best for last, wait on this one. In summary KotN

From a role-playing perspective, I would save KotN for last because:

1. You HAVE to be good (otherwise you can't do the quests) and you must rid yourself of your infamy
2. Epic boss fight (gorgeous, but not really very challenging)
3. Finishing the quest line starts your...uh...ascension (trying to avoid spoilers here) to a...higher place maybe? (but don't worry, it is purely ceremonial; the game does not end).

I feel like an aging warrior, mage or assassin (whatever your playstyle) may realise that he or she has made many mistakes and bad choices in life, and with KotN sees a way to finally do some good and die at peace.

Cool, thanks for all the input! For now, I'd say this confirms I can go ahead with completing the Main Quest (after finishing "The Collector" related quests) and see if I can pick up other quests along the way.

Remember that you don't have to complete any quest line before starting another. I'd vote for doing them all at the same time.

There are a few "can't this at the same time as that" situations to be aware of, but not many. Just don't kill anybody you'll need later!