I did a quick survey of my collection of unpainted Inquisitors and I discovered I have over twenty-four of them, ranging from Rogue Trader era to the new ones like Eisenhorn and Greyfax. So, I decided to do a back-and-forth and am going to paint the oldest, then the newest, until I meet somewhere in the middle.
I started with the earliest release I have unpainted, which was the famous Obiwan Sherlock Clousseau from the original Rogue Trader 40K book. I had painted one before, many years ago, but nowhere near this well.
I believe Obiwan Sherlock was the first named Inquisitor. I recall being perplexed by his weapon, and I have to say I'm still not sure exactly what it is supposed to be. I think he has a power sword in his description, but he is labeled as 'Lasgun 1' in an old Citadel catalogue.
I ended up deciding it was a xenos weapon and used a small piece of a Necron decal to add some subtle circuitry.
The next Inquisitor will be the most recently released: Gregor Eisenhorn (sadly, Finecast, but still quite nice).