Hi,
I also voted for Longleat. I have been to all 3 and oasis and I think Oasis is the best. Out of the CP resorts I think Longleat is the best. It is very scenic and well-kept and is the most up-to-date and spacious of them all and has the best aqua sana leisure complex.
If you live nearer Oasis, I would choose to go there, the gorgeous detached log chalets are beautifully eqipped and have so many amenities including a jacuzzi and stunning kitchen, plus Oasis is set in the Lake District and is even more stunning and beautiful than the other CP resorts.
But overall, you will not be dissapointed with Longleat, especially if you are going for long enough to actually enjoy it. At all CP resorts good weather is a massive bonus! But a weekend isn't enough in my opinion, I think 5 days - 1 week is a nice way to enjoy Center Parcs.
Richard.