If you like an historical angle, I’d recommend Clive Cussler’s Dirk Pitt series (he’s expanded into other series with co-authors, but I’ve only read a few of them).
Personally, I’m nearing the end of Westlake’s (as Richard Stark) Parker series. These are like potato chips. Very easy to pick the next one up right after finishing the prior one.
I’ve also been working my way through the Matt Helm series. Very enjoyable, though I tend to go in spurts of three or four at a time.
Oh, and I should have mentioned Cussler’s newest series, Isaac Bell. I have read most of those, and they’re good fun. Unlike his other series, they’re actually set in the past.