Most Strategically Successful M&A deals of the last few years?

GSK buying Tesaro in 2018. Pretty much bought the company for niraparib (Zejula) which is going quite well as the 2nd PARP inhibitor to market. They have benefitted from strong reimbursement across the EU5, good access in all major US health plans and a solid pricing strategy across geographies. They will likely continue to grow and move further up the treatment paradigm (as well as expand across other indications). This acquisition was crucial for GSK who made some poor decisions in the years prior and needed something to prop them back up.

Airbus buying Bombardier's C-Series in 2018. Not only was this is very very clever move from a import tax avoidance standpoint (as part of the C-Series plane is produced in the US) but also a great move to capture a portion of the small plane market which has historically been dominated by Bombardier and Embraer. Sure Embraer is in the process of being bought by Boeing but there is still no sign of when that transaction will close. The A220 is a phenomenal plane and this has been reflected in their orders (including a sizeable one from Delta). In comparison (and accounting for time difference), Embraer's new E2 variant of the E175, E190 and E195 has had a much smaller number of orders. I think Airbus chose very well with this acquisition

I'm sure there are plenty more, but these are the first to come to mind.