Best is a matter of taste of course but Alec was the original. Hunt for Red October was the 1st Clancy movie even though Patriot Games actually takes place earlier in the Ryan timeline. I think part of the reason I don't like Ford as Ryan was because his two movies sucked in comparison to HFRO. Patriot Games was okay but Clear & Present Danger was a god awful adaptation.
I'm an avid reader so these book to movie castings are always kind of intriguing. I saw Sahara before I read any of the Cussler / Dirk Pitt books, so I always just kind of pictured McConaughey, Zahn & Macy as Pitt, Giordino & Sandecker as I read through the series.
The Lee Child / Jack Reacher books are another favorite of mine. They have pretty much all been optioned for the big screen but Child has rejected every Reacher casting so far. (The Rock most recently).
Back to the original topic.....A Bourne movie without Damon would be stupid. That being said, I have yet to actually start reading Ludlum's Bourne series.
The Vince Flynn / Mitch Rapp series and Brad Thor / Scot Harvath series would make great movies also....If Hollywood didn't fuck 'em up like they usually do.