I've only read the Isaac Bell novels. I'd start with the first book, The Chase. Part western, part noir mystery, part retro-techno thriller.
But it depends on what you're into.
The Dirk Pitt adventures are a cross between James Bond and Indiana Jones. Lots of sea based capers and adventures. Start with The Mediterranean Caper.
The NUMA Files are a spin off series of books in the same universe. Serpent is their first book.
The Oregon files is more espionage, heist and spy thrillers also in the same universe. But they aren't introduced into the Dirk Pitt book Flood Tide and starts with it's own book, Golden Buddha.
The Fargo adventures are pulp style treasure hunting adventures. That starts with Spartan Gold.