"I'll Take You There" by The Staple Singers (bass by David Hood) because it's a simple little groove, but it's the song. Also because it's the glue for all the solos. I use it as a mantra-like practice routine now. Can you play that groove for five or ten minutes straight with consistent attack, time, inflection, etc. and resist the urge to embellish? Only then do you get to do the cool little solo in it.
"Good Hearted Woman" by Waylon & Willie. I hated that song for a long time. When I started playing, I knew I'd have to play it every night, sometimes two or three times a night. It's dirt simple country Root/five with a single walk-up from I to IV. There's a key change, but it's the most annoying kind ever. I finally figured I could either hate those three minutes of my life, or learn to embrace them. I chose to learn to like it. It became my nightly bass lesson. Focus on the precise attack needed, the exact timing of the kick drum, when to release so the snare came through, the tone I wanted to get from my fingers, how to lock exactly with THIS drummer, etc.
"I Want You Back" by The Jackson Five (bass by Wilton Felder), because it made me sharpen up my timing, work out some awkward fingerings, and lock with both the drums AND the piano. Bonus points because our singer needed it in F instead of the recording's Ab, and I didn't have a five-string then.
John