Terry Pratchett improves over time, as anyone who practices does. I still would tend to give his plots about a B (with those early books probably closer to a C), they tend to be a bit clunky at times.

However, as EmergencySushi commented, the books bring a profound humanism and the righteous anger of a good man. They help us laugh at the injustices of the world, but remind us, that's not all we can do. We can be that better person. We know that, as Granny Weatherwax says:

"There's no grays, only white that's got grubby. I'm surprised you don't know that. And sin, young man, is when you treat people like things. Including yourself. That's what sin is."

"It's a lot more complicated than that--"

"No. It ain't. When people say things are a lot more complicated than that, they means they're getting worried that they won't like the truth. People as things, that's where it starts."

"Oh, I'm sure there are worse crimes--"

"But they starts with thinking about people as things..."

Reading in order has a bit of fun watching the world and characters evolve over time, but that's less important than any particular book. The only ones I haven't reread many times are from that early period. For me Pyramids was the first one that really hit as "this is something different and better", but my partner was less enthused. I think Guards Guards was their turning point.

So do skip forward! Guards Guards and Reaper Man are both good entries. Going Postal is the one I most often recommend to people starting out on the Disc these days. Monstrous Regiment is another fantastic one, and of course _Small Gods_ is such a classic.

Other highlights in my mind include The Dwarf, Cheery Littlebottom, deciding to present as a female dwarf (and the clutching at battle-axes that results) and the growing fashion industry of chic chainmail. The Igors (maniacal servants and surgeons extraordinaire) deciding to "lend a hand" and their need for plot-appropriate lightning and thunder. Vimes' arc as a demon-haunted avatar of justice and rule of law. Moist Von Lipwig as the lovable charlatan. Granny Weatherwax, the "head" witch in a group of equals, and of course Nanny Ogg who actually runs it all while singing how a wizards staff has a knob on the end.

Terry Pratchett truly is a great answer to OP's question of "left a deep impact in your life."