Avoid useless enhancers, such as, extraordinary, fantastic, great or superb; demonstrate such qualities with your prose.
A sample from the House & Garden cable network (that indirectly inspired this page).
Instead of saying a piece of cheesecake is great, the speaker achieves the same aim with specifics:
This piece of cheesecake, it's moist, it's light, it's got a touch of amaretto and a crunchy graham-cracker crust.
The ultimate useless enhancer is very. The original meaning of very is in a high degree; extremely. Frequently "very" is unnecessarily used to intensify an adjective.
a very high value -> a high value
a very bright person -> a bright person.
Don't offer an opinion; instead let the facts speak for themselves. Saying something is the "best" naturally causes the reader or listener to mentally (or, worse, vocally) disagree.
In giving a talk, you want the audience to trust you. Do this by being concrete and straight-forward, not opinionated.
Special relativity is the best theory of space-time.
Special relativity, in a single theory, reconciled the experiments that detected no motion through the ether and, while maintaining the concept of coincidence, provided a quantitative relation between the space-time coordinates of objects moving relative to each other.
A negative statement doesn't exist that can't be put in a positive way. This is not recommendation to be blindly optimistic, but rather to stress the positive achievement.
Heisenberg, in his PhD thesis defense, didn't understand the resolving power of a microscope. What a blockhead!
The initial difficulty Heisenberg had in understanding the resolving power of microscope led him to deep thought about how to simultaneously measure the position and momentum of a particle (for example, with a microscope), thought that led to the uncertainty principle.