When you're in a bad mood, what games always seem to improve it? Whether through cathartic destruction or cuteness overload, what brings you joy? Whole games, segments, minigames, whatever.
The Chao Garden from the Sonic Adventure games, especially the second one. Between the adorable little chao themselves, the tiny animals you collect for them, the impossibly sweet music, and the serene scenery (aside from the evil garden, of course). The entire setup is pure sweet dream fuel, and the fact that it remains paused when you don't play it keeps you from getting stressed out about having to get a schedule like other virtual pets.
When I just want to wreck everything, nothing beats the sheer catharsis factor of Koei's Warriors series games. Load up a map with no real objectives, grab Lu Bu, and sweep thousands aside!
If I'm particularly bloodthirsty, I can never go wrong with Brutal Doom. RIPPAN AND TEARRAN.
Dark Cloud 2 (or Dark Chronicle to you in PAL territories) is great for escapism. There's an absurd amount of content to get lost in. Build a town! Fish! Explore procedural dungeons! Fish in the procedural dungeons! Play golf-disguised-as-fixing-time-distortions! Build and customize a robot! Takes pictures of nice scenery! Race fish you've caught! Dress Monica up like a catgirl! There's lots of good music, to boot. The song in the Rainbow Butterfly Wood is so soothing. Just…don't try getting "scoop" pictures unless you're feeling super-patient.