There are different ways to approach giving a 25th anniversary speech (this anniversary is referred to as the silver anniversary). One way is to start with a quote, such as the proverb "a good wife and health are a man’s best wealth." Another possibility is the Oscar Wilde quote "a good marriage is like a casserole; only those responsible for it really know what goes into it." You could start with a quote and then speak about it.
For example, if you started with the proverb that a good wife is a man's best wealth, you could include anecdotes and stories about the ways in which your wife has supported you in tough times and has supported the growth of your children. You might want to include a mix of funny and serious anecdotes. Perhaps start with a funny story about a way in which she helped your children when they were little and then move to a more serious story about how she supported your family in difficult times. Consider including the things that connect you and your wife and that you enjoy, such as hobbies, sports, or trips you've taken. In the end, thank her for her love and support and perhaps mention that you are looking forward to another 25 years of marriage. You should end by asking everyone to stand up for a toast to your wife.