Casio fx-991s. I've been using one (well, actually, this is the second) since 5
th or 6
th grade. It's out of production, and lacks some of the more esoteric features, but the layout is good, it's compact, and quite durable. It can't do differentiation or integration, and it doesn't have the equation line above the mantissa; otherwise, it's pretty much the same as any other good scientific calculator. I never much liked the TI interface on the graphing calculators; unfortunately, it's pretty much the only game in town. At least the new models are fast enough that they don't get annoying for simple tasks. I've never owned one, though, unless the TI-83+ emulator software counts. Now, if only I could get it to run Wolf83.... (That's essentially Wolfenstein 3D for the calculator.) If I'm on the computer, I tend to use GraphCalc; it's full of little bugs, but works very well for simple (scientifc) calculations and, as its name implies, 2-D and 3-D graphing. I end up using Mathcad, much as I don't like the interface, for solving more complex problems on the computer.
Edit: This one....
