What is the best method for cooking Turducken?

Welcome to Cherfalk, Hellcat.

I'm not so sure it would hold together on a spit, so would opt for the conventional oven.

Are you making your own from scratch, or using a store-bought one? If the latter, many of them are pre-cooked and just need reheating. So the conventional oven makes even more sense in that case.

Well if you cook anything incorrectly it is dry 

I am not a big fan of injecting I think you end up with a watery mess. My preference would be to brine it a bit and then slow cook it. This is not something that was meant for a spit so I would not go that route. Like Ky said it would just fall apart unless you really tie it on there. I think slow cooking it really would turn out the best product. If you have a crock pot put it in there and I think you would get the best product.

Nicko, I don't know about the brining since she is buying it already put together.  Doesn't a turducken normally contain stuffing?  If so won't that stuffing get overly waterlogged if attempting to brine it?  But I do agree, wholeheartedly with the slow and low method.  I don't know that I would cook it above 250°F, then once I come within a few degrees of where I want it to be then blast it for a few minutes to crisp up the skin.

A glance at Turducken Cooking Directions might be helpful

Man Pete I don't know if it is stuffed. I was never a fan of the idea because I think no matter what you do it will always end up dry. So if it is stuffed I would not inject or brine then.

I am telling you drop that baby in the slow cooker, let it go and enjoy a good box of wine. 

Nicko, far as I know, turducken is always stuffed. There's a layer of it placed between each bird as you stack them.

I agree with you; no matter how they're cooked the tendency is for them to be dry. Of course, most turkey served on Thanksgiving is dry anyway.

What impresses people is the idea of three different birds (whose flavors, in my opinion, don't meld together well anyway) in once slice of meat. Like all gimmicks, it's the gimmick, itself, that people relate to. The flavor and degree of cooking get lost in the shuffle. 

But it doesn't have to be that way, even with something as big and dense as a Turducken.  It just takes some finesse and going slow and low.  Unfortunately, people don't want to spend the time or energy doing it the right way most of the time.  That's why the end up with turkey that is in serious need of gravy to be eaten.  I save the gravy for the potatoes and stuffing as I haven't served a dry bird in my memory.

I will agree that there must be a way to cook a Turducken without drying it out. I will also agree that the way it is constructed, getting the chicken in the middle cooked without drying out the turkey on the outside would be a difficult task. After all, just roasting a stuffed turkey to the point where the stuffing is cooked to an adequate temperature and the turkey is still tender and juicy is an almost impossible juggling act. I never stuff my turkeys; I put my "stuffing" in a glass baking dish and put it in the oven when the turkey comes out and is resting.

I will also admit I have never attempted to roast a Turducken, nor do I think I would ever want to. I agree with the earlier post that says the novelty mostly outweighs the actual quality of the meal. And as I said, I have never cooked one, but I know a bit about roasting birds. However, if someone were to put a gun to my head, and insist I cook a perfect Turducken or else, I would preheat my oven to 250 degrees (for about an hour) stick my thermometer's probe all the way into the chicken portion, put the Turducken in the oven, and then wait until the thermometer read about 155 degrees. I would then crank the oven to about 450 to crisp the outside of it, and pull it out when the thermometer read 165 degrees. I would then let it rest, and finish up my side dishes.

At 250 degrees, this would take a lot longer than roasting a normal turkey, but I think the results would be superior. In my experience, injecting and basting are largely useless. Even if you inject poultry stock or real butter, the results are most often less than spectacular, and basting pretty much ensures a soggy skin and vastly increases the cooking time without adding any juiciness to the meat.

Another option I might consider is to deep fry it in peanut oil. At the proper temperature, the oil will work to hold the moisture in. However, I would not be sanguine regarding it holding together while doing so. I would have to make sure it was properly trussed first.

Of course this is all theoretical. I would dearly love to hear from someone who has experience cooking one. One that turned out well, that is. :)

Cornelius, I don't think frying would work in this situation as with a turducken you have a very large, solid mass as opposed to a turkey that has the open cavity.  This an item this large and dense the outside would overcook before the center is even close to being done.  And again, you are also dealing with stuffing that would absorb a lot of oil and become really greasy.