Well, I just tested this out after being bummed that Dolphin had more requirements than I thought. I went through most of my (small) available PS2 library, and the performance sucks on most of them, or have too many graphical glitches to enjoy. Unfortunately I don't remember the plugins and settings I used for each.
*MGS2: Heavy slowdown, Snake is invisible in gameplay (codecs and cutscenes work okay)
*MGS3: Slowdown, slow speech, black spots where light spots on the forest ground should be,
some missing GUI layers on the codec screen
*Silent Hill 2: Half-screen on one plugin, massive slowdown on another.
*Silent Hill 3: Almost, but not quite, better than the above. Also strange shadows.
*Siren 1: MUCH better using ZeroGS, but now the videos look interlaced, and parts of the level intro screens stick into the gameplay world (like some kind of strange virtual burn-in lol)
*Resident Evil CODE: Veronica X: Worked great in DX9 software(?), until the explosions and enemies appeared.
*Final Fantasy XII: FINALLY, something that almost works great! Just as beautiful as the screenshots suggested, and very fluid so far. Of course, it's also FFXII, so I'm not sure if I actually want to play through again, enhanced visuals or not.
So what are the best-performing games under PCSX2 so far? Is there a list of recommended settings for the above games?