Hello me from the past! I recall making the same exact forum post with the same exact answer.
I could easily say the HP Prime, as it's a modern calculator with a (for a calculator) blazing fast 400Mhz ARM processor, colour touch screen, and good implementations of giac and RPN, along with a great suite of graphing utilities that fully take advantage of the touch screen.
However I'm not going to say that it's my favourite, and while it's definitely the most impressive one I own, I have to give that trophy to the HP 50g, which has one of, if not the best RPN implementations available. RPN CAS with textbook notation! It's amazingly fluid too, especially if you are used to RPN, and it is one of the rare instances where I think a calculator is actually fun to use.
The HP 48g, which is my only other calculator, takes third place, but shouldn't be considered as bad either. For the time it was available, it was easily one of the most impressive calculators around, and I wish I had one of the variations that had CAS, as I'm wondering if it has the same system the HP 50g has (making it both historically and visually interesting, as well as crazily useful).
The answers you will get, however, depends on who is doing what. HP calcs have been a fan of EEs and more power users for decades, but almost everyone on the planet has had the misfortune to use a Z80 based Ti calculator, and a lot of people only think of those as the best graphing calculators can be, which just annoys me. I am actually rather interested in the 68k calcs (as I love the 68k) but I don't expect those to be particularly impressive compared to a Saturn or ARM based HP calculator of similar vintage.