Which one should be prioritised Revenue, Cost or Financial Synergy ?

There is no fixed answer to this question. It all depends on the rationale or strategic intent of your acquisition. If you acquire your competitor with very strong overlap in product portfolio and customer portfolio, you probably go for cost synergies and market synergies (= increased negotiation power against suppliers and customers). If you acquire a company to extend your product portfolio, you will probably focus more on revenue synergies (e.g. cross-selling).

The best and ideal opportunity should offer a combination of the three, but this is rarely the case. I agree that it all depends on the rationale or strategic intent of the acquisition. It also depends on the quality of the due diligence. Sometimes revenue synergies are overestimated and then the focus quickly shifts to the cost synergies and ultimately financial synergies when the deal structure is impacted.

Having cost synergies overestimated also happens quite frequently and usually the integration that follows is rocky as it puts both companies under significant pressure.