Brut: Anything that can be reliably verified later would work for most people. Especially where it's unknown to you, the server admins, prior to the win. Info from the blockchain like the latest block is probably more reliable (although guessable to an extent) than transaction ID. Depending on the frequency of the draw though you could use something else, such as the lotto numbers from a national lottery draw. Or you could ask every player to enter some data, such as a random 3-digit number, and combine the inputted numbers at the time of the draw and use that to influence the maths so the draw is provably fair...
Whatever you do - the simpler the better. The more easily people can grasp it the more likely people will trust it. I look at some of these provably fair ideas that almost need a Phd in statistics to decipher and I just shake my head
