I don't remember exactly which calculator I first had but it was very much like this and my "big" Christmas present in mid 1970s this is the HP-35 Calculator from 1972. Look at the cost! It is still my favourite calculator of all time. Great keyboard, and used Reverse Polish notation logic for entry, which seemed so logical.
HP 48GX by Hewlett Packard. I picked this up at a local flea market for 25$. It goes on eBay for upwards of 100 bucks! I love the HP's reverse polish notation. If you haven't given an HP a try, do! Next up, HP 50 GX.
The HP-15C Calculator Scientific Calculator was an RPN (Reverse Polish Notation) calculator that was popular with engineers back in the '80s, and it was manufactured from 1981 through 1989. It was a programmable calculator that had built-in support for complex numbers, matrix math, numeric integration, and root solving. It's small size (5” x 3.15”) allowed it to be carried in a shirt pocket.