Example 1
The best thing I have ever done was/is come to Australia.
What are the differences in meaning?
Example 3
The only gambling I have ever done was/is playing Roulette.
What are the differences in meaning?
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Sign up to join this communityThey're both grammatically correct.
The difference in meaning between them is very similar to the difference in meaning between simple past and present perfect. The simple past refers to a finished event in the past without any other nuance, while the present perfect refers to an event in the past to give information about the present.
So, in your first sentence, since the person is in Australia at the moment of speaking, there is a clear present result, so "is" makes a lot more sense.
In the second example, "was" suggests that the speaker no longer gambles, while "is" suggests that the speaker may still gamble.
That said, both "was" and "is" are technically correct in both situations, just not necessarily the more natural choice.