I look all the Cr cards available on the market and i cant say for sure its worth spending clintz on. Some are very weak in terms of attack, damage, ability, or bonus. Some are the exact opposite. When I look at the price, i think " is it worth spending millions of clintz on?" Look at the prices these days: its anywhere between 10000 to over 16 million.
Tell me what you think.
If you insist on investing, pick a rare card that doesn't have terrible stats. Blaaster, Heegrn, Caelus, Jackie...
If you are concerned with how well a card performs in the game, I would say none of them are worth spending Clintz on.
Strongest CRs are all penalized heavily in DT and banned from ELO. Good CRs that were former staples now have great replacements, for the most part.
If you're talking about investing, go for powerful CRs in the mid price range like Jackie Cr och Tanaereva Cr. Those are pretty much bound to increase in value over time.
But if you're talking about making a powerful deck, there aren't many CRs that are expensive but also relatively balanced. The ones I can think of are Tessa Cr, Splata Cr, Marlysa Cr and Nahi Cr.
Almost all CR's are good picks, the difference is just how fast that CR will rise relevant to other CR's.
For the simple people who want to do easy buys: Less than 100k, rare or uncommon and good on the playfield = good enough.