"Good Lovin'" (The Young Rascals)

The Rascals (or "The Young Rascals" as they were known for a few years for legal reasons) pretty much defined the genre of "blue-eyed soul."  The New York-based band had a sound that was equal parts R&B and garage rock, with even a touch of jazz.
Perfect specimen is "Good Lovin' (1966)," which actually is a cover of a 1965 tune by The Olympics, a doo-wop/R&B group best known for novelty songs (like 1958's "Western Movies," which is replete with campy gunshot sound effects).
Story goes, keyboardist/vocalist Felix Cavaliere heard The Olympics' Latin-tinged version on R&B radio in New York and immediately decided that The Rascals had to cover it in their live shows.  
Then, when the band landed its recording contract with Atlantic Records, they cut the song in the studio for their covers-heavy first album, The Young Rascals.  Although the band members weren't too pleased with the results, feeling the final product sounded sloppy, the experts behind the mixing board, producers Tom Dowd and Arif Mardin, convinced them that it had the tight-but-loose sound that would make it an instant hit.  (It went to #1 on the Billboard charts in April 1966.)
Being a keyboardist myself, I always gravitate to Cavaliere's Hammond B-3 solo after the second chorus.  Honestly, there's nothing complicated about it: a few sustained notes over some block chords; he's basically just replaying the horn break from The Olympics' original.  But there's something soul stirring about the resonating, church-y sound he conjures from that electric organ.  It brings to mind jazz organist, Jimmy Smith, who definitely influenced Cavaliere's sound.
And, although it's gimmicky, I still love that false stop at the end of the song, where you get that little bonus reprise of the chorus and Cavaliere sounding like he's having a religious experience.  

"How Can I Be Sure?" (The Rascals)

The Rascals (or The Young Rascals as they were called for legal reasons early in their career) had one of the most distinctive sounds of the 60s.  No other band combined garage rock grit with jazz chops quite the way they did.  
Along with Booker T. & The M.G.'s, the band made the Hammond organ a viable instrument for rock & roll.  I'd even contend that, without The Rascals paving the way, an organ + guitar-centered band like The Doors probably wouldn't have gotten a recording contract.
The album Groovin' (1967) is a particular standout in the band's catalog.  In an era known for albums packed with filler and maybe one or two hits, Groovin' contains exceptionally strong tracks that effortlessly blend an array of influences and styles: San Francisco psychedelia, British pop, East Harlem funk, Uptown jazz, and Motown/Philly-inspired soul.  
One of the key songs from Groovin' is "How Can I Be Sure?", a memorable ballad co-written by band members Felix Cavaliere and Eddie Brigati.  Despite its optimistic tone, the song was inspired by Cavaliere's growing doubts about his engagement to his girlfriend at the time.  (Spoiler: they split soon after this song dropped.)

It's worlds apart from the band's earlier output, like the scorching "I Ain't Gonna Eat Out My Heart Anymore" or absolutely pentecostal "Good Lovin'."  Yet there's still that foundation of New York City R&B that runs deep in its veins.  You can hear it in Brigati's lead vocal as he sings about love and uncertainty over the Parisian accordion and lush orchestration.  

It's a song that defies easy categorization, but it's unmistakably The Rascals, through and through.