koima57: Dolphin is my favorite emulator of the 3 I use; PCSX2 and PPSSPP are the other ones.
dtgreene: For me, the problem with those emulators are that they are for systems more modern than I generally am interested in emulating.
In fact, I have an informal "two generation" rule for emulation; I won't emulate commercial console games unless they're for a system that is at least 2 generations old, and PPSSPP is not capable of emulating such a system.
Anyway, I happen to like mednafen since it can run on the Linux console without having to start X, and it doesn't need a mouse. One thing I don't like about mednafen, however, is that it asks you to assign keys to turbo buttons, and I would rather not do so. (The games I play don't need them (and in the rare case that it does, I consider it a flaw in the game), and I prefer not to use them, a d sometimes I might be using a controller that doesn't have any spare buttons for turbo.)
DOSBox is another emulator I like to use. One issue is that I can start it from the Linux console (no X), but the keymap is all messed up when I do that, and I can't even start a game or even type the "exit" command from the prompt. Also, some games that run under it use the mouse, and the one on my laptop isn't the most reliable.
And they do necessite a powerful processing computer, and often case-by-case emulator settings to run correctly.
I use them for convenience, not pirating. Full HD screen for my PSP games instead of the awful Vita Tv result, 1080p instead of the poor, grainy 480p of my native Wii and ditto for my blury Ps2 screens.
I use also PSP custom firmware (CWF PRO 6.61 iirc for my needs) and one NDS flashcart (Supercard Ds Two) for my backup copies; my only rule is to never pirate anything anymore for now 6 years, without regret or temptation.
Oh by the way of emulators; ScummVM is great too!