HP-28C HP-41C HP-16C
I have a Sharp EL-5806 (LCD Scientific Calculator) that had roughly the same functions as the old HP-35, but I made it all the way through college on one set of button batteries. It cost about $30 while the HP-35 was still selling for a few hundred.
But my favorite for quickie calculations is a Post Versalog. I figure that if I ever find a Curta in nice shape for a reasonable price, those two will be all that I'll need.