American Dreaming by Dead Can Dance
“I Just Want to Celebrate” - Rare Earth, a guy has a crappy day, but still feels good.
I don’t know why but “No Rain” by Blind Melon always struck me as a really happy feel good tune.
“Saturday, In The Park” by Chicago
"What a Wonderful World" as sung by Louis Armstrong. I’m sure that title is wrong, but you all know what I’m talking about.
Adam Ant’s Goody Two-Shoes always raises my spirits whenever I hear it. It’s near impossible for me to hear that tune and not feel happier.
Right by Your Side (the Eurhythmics)
Stand by REM.
Good Morning Sunshine - Oliver
Don’t know the title or who does it, but it’s the song that goes “If you wanna be happy for the rest of your life, never make a pretty woman your wife…”
Barnes and Barnes did a killer version of it, but I don’t have a clue who did the original.
In the Summertime by Mungo Jerry.
Girls Just Want to Have Fun by Cyndi Lauper!
and from an earlier generation, Collegiate by the Fred Waring Orchestra.
almost forgot So in Love by O.M.D.
59th Street Bridge Song (Feelin’ Groovy) by Simon and Garfunkel. Now THAT’S a happy song.
Island in the Sun by Weezer
“Beyond the Realms of Death” by Judas Priest
“Black Celebration” by Depeche Mode
“Seasons in the Abyss” by Slayer
…well, they make ME happy… 
Second “59th Street Bridge Song”.
“Jackie Wilson Said” and “Brown-Eyed Girl” by Van Morrison.
“Seasons of Love” from Rent.
“Solsbury Hill” by Peter Gabriel.
Knew I forgot to add something. IIRC, it was Jimmy Soul.
“Victoria” by the Kinks.
I especially like the part where Dave Davies screams with what I believe to be happiness and good spirits.
The Way by Fastball
Another Night by MC Sar and the Real McCoy
Sunshine, Lollypops and Rainbows by Lesley Gore
Most Christmas Songs (especially by Disney!!)
2nd nom for Mr Blue Sky by ELO
I nominate Last Thing On My Mind by Steps as the happiest sounding song with the most dismal lyrics.
Glad All Over by the Dave Clark Five is pretty gosh darn happy.