As stated by the previous posters it depends on the level of the rider and horse as to which sex is best. There are more risk with stallions. Mares can be moody, even geldings can have their days. For novice I would say for safety it would have to be a gelding or mare. Stallions are best left to a more experienced person no matter how nice they are. Even the best stallion will have a lapse of good judgement from time to time.

Hope this helps.