I agree with the others, as it depends on the horse. But the ones I have had the most luck on in eventing are thoroughbreds.
My successful rides have all been raced but it depends on a couple of factors. The horse has to be in good nick no injuries, or aliments such as bleeding from the nose when racing. That's the main dodgey thing I come across with ex racers and people trying to get rid of them.
The advantages to an ex race though are that most will be confident and already trained in the show, noise and spook scene if they came from a decent trainer. They will be fit and usually in good condition. They will be speedy and agile... always good for Xcountry. Usually will have good feet, again depends on why the horse is being sold and it pre purchase check. They also usually have good footing for the tricky parts of eventing and dressage. They are use to the training, handling and floating.
I have one at the moment who is an absolute gentlemen and one of the best jumpers I know. He has the confidence, so you can put anything in front of him and a good rider on his back and he is over it beautifully. He is comfortable in the show scene as well which is always an added bonus as you know he will perform well.
I guess it depends on your own preference though. I like the ex racers as they have personality and they enjoy work. You just have to pick and choose the right one that's all.
Thoroughbred in general have always been my favourite for competition, good personality, smart, confident, good looking, agile, speedy, and usually well rounded when it comes to skills and training. You can usually pick up nice little bargin too if you do your research. :o)
Gerry was a power house for eventing, the tb in him gave him speed and the smarts, clyed in him gave him the ability to plow through any thing, we also used gerry for fox hunting, he was a good all round horse, another breed I like is the canadian draft