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Let's say that two people are talking about an invention which is amazing in that it is beautiful or proves an impossible concept but hardly has any use. How can one refer to it in a sarcastic manner, as if saying that the invention was the need of the day ?

Sample Dialogue -

Thawed out caveman : I must say the science of your times is quite
amazing. I never quite could have imagined something like this.

Modern day scientist : Yes...Modern technology has grown by leaps and
bounds...We have X, Y and Z

Thawed out caveman : And then there is cloning

Modern day scientist : Yes, truly one of our "_________"
technologies in this day and age of overpopulation.

Invaluable has the potential for sarcasm. Most dictionaries will record its primary and most commonly understood meaning:

: valuable beyond estimation : priceless

However, note two things. First, In- is a prefix that negates what follows. In conventional use, invaluable means that it can't be valued. Yet a second meaning of the word also exists: it has no value. From the Oxford English Dictionary:

2. Without value, valueless.

Here is an example from a 19th century play John Bull by George Colman:

I flattered myself I might not be altogether invaluable to your Ladyship.

People have played with this word to create sarcasm: sure, the superficial reading is that they're beyond value, but something about the tone and context suggests that they're not valued. A couple of other uses that strike a sarcastic tone:

@ disney, thank you for making me, as well as all of my PhotoPass coworkers and friends, feel replaceable and invaluable. ... Saying I’m invaluable is sarcasm.

They say sarcasm is the lowest form of wit. Yeah, right. Assuming "they" are the same experts who tell us "Look before you leap" and "He who hesitates is lost", then I think we all know how incisive and invaluable their advice is. What would we do without it?

So your jaded modern scientist would be engaging in wordplay while subtly hinting at their true disposition towards cloning.

You might go for straight out sarcasm with productive.

Yes, truly one of our most productive technologies in this day and age of overpopulation.

On the one hand, it is used to describe something good:

2 achieving good results

    a productive partnership

    Politeness is usually more productive than confrontation.
_Macmillan

On the otherhand, it can just mean making something:

3 FORMAL causing or producing something
_Macmillan

How about "most precious" or "most darling"? Or "sterling"? These words are all a little over the top — you know right away that if a technology is truly wonderful, you would never call it "darling."

This is what white elephant means.

white elephant n

1. An expensive item that is troublesome or useless. The term comes from a story about the king of Siam, who was said to have given an albino elephant, considered sacred, to a member of the court whom he disliked, knowing that taking care of the animal would exhaust the person's fortune. At first, Eve was excited to inherit the farm, but it soon proved to be a white elephant she couldn't afford.

^{—The Free Dictionary Online}