The Firm' absolutely has to top any list. That's the one that pulled me into legal thrillers to begin with. It's just so tightly wound; you can feel Mitch McDeere's panic as his dream job turns into a gilded cage. The sheer paranoia of being watched all the time, the impossible choice between the FBI and the mob—it's a masterclass in sustained tension. I reread it last year and it still holds up.
After that, I'd argue for 'A Time to Kill'. The raw emotional core of that story, that opening scene, gives the legal maneuvering so much more weight than just clever courtroom tricks. It's Grisham with heart, not just plot mechanics. 'The Pelican Brief' is a close third for its sheer breakneck pace; it feels like a movie (and, well, it became one). Some of his later stuff blends together for me, but those early 90s novels are untouchable.
Grisham's skill is in making complex procedures readable. 'The Appeal' is a brutal, cynical look at how judicial elections are bought. It’s not his most beloved, but it’s arguably his most important politically. It’ll make you angry in the best way. Start there if you want substance with your suspense.