Whats your wife's favorite position to take a big thick cock

For me, definitely start by sitting on top, slowly feeling my way down him inch by inch, teasing him but also pulling back if it is too much. I love that feeling of control but also the knowledge that there is more to come as I get wetter and more relaxed. I love having my long nipples sucked as I do that too.
Having had over-enthusiastic and inexperienced lovers who thought that an immediate pounding was what I needed (usually younger men in my past who thought a massive cock was all that mattered), it's much more fulfilling for me, especially if they are very thick and inexperienced as I'm not into a 3 minute pain session!!
Having that control is not only hugely sexy but helps me relax enough to move on to other positions where a man can be more forceful without damaging me as I'm usually extremely wet and swollen by then and can concentrate on really feeling him deep as well as his girth.
That's what works for me, I guess that dominating an otherwise over-confident and hugely-hung lover is not only physically better but psychologically much more enjoyable and horny for both of us.
Having said that, since I've been married it has been a few years since I was in that situation so I may react differently if given the opportunity ;)

This was always my preferred way to manage and ultimately get the most enjoyment with a lover who was very well endowed, and it involves psychological teasing and some initial domination whilst knowing I would submit and let go when ready, but it may not be for everyone.