IMHO:
- Gimme Some Lovin’ - Spencer Davis Group
1A. Higher and Higher - Jackie Wilson
Your suggestions welcome.
Oh, I thought this was going to be Oingo Boingo
Who have made any number of feel good songs.
https://www.youtube.com/watch?v=wvUQcnfwUUM
Mungo Jerry
Katrina and the Waves -Walking in Sunshine
Mouth and MacNeil -How Do You Do
Its gotta be 59th street bridge song (Feeling groovy) Simon and Garfunkle.
All is groovy!
That’s the Way I Like It, KC and The Sunshine Band
Come On Eileen, Dexy’s Midnight Runners
99 Red Balloons, Nena
Girls Just Wanna Have Fun, Cyndi Lauper
Oh, and Footloose, Kenny Loggins
Everytime You Touch Me - Moby
Everybody’s Free - Rozalla
Let Me Be Your Fantasy - Baby D
It Feels So Good - Sonique
Dub Be Good To Me - Beats International
Some elderly standards.
**Sir Duke **- Stevie Wonder
For me? Probably “Top of the World” by the Carpenters. Karen’s voice is like a warm blanket.
Not sure that a song about nuclear apocalypse is the ultimate feel good song.
I still like it though. 
I like to go old fashion here:
Joy to the World (Jeremiah was a Bullfrog.)
As with so many musical questions, the correct answer is, of course, Louie Louie by The Kingsmen.
Mr. Bojangles, especially this version:
wouldn’t it be
Don’t Worry, Be Happy
Seconded. This was my first thought when I read the thread title.
Obviously, “I Feel Good” by James Brown. 
I Can See Clearly Now by Jimmy Cliff. What a happy, uplifting, pleasant song.
Pretenders - Don’t Get Me Wrong
First song to come to mind: Good Day Sunshine.
Those are hard to beat - two of my favorite songs.
I also like Everlasting Love with Carl Carlton.