(Edited with more research done - thank you for the suggestions/corrections!)
The stationary Braivary (White 2) and/or Mandibuzz (Black 2) are a fantastic option for powerful Pokemon available quite early in the game (Route 4). Lillipup is incredibly useful as a HM slave, and a few others with great potential attainable very early on, with the exception of Riolu, are:
- Eevee
- Zorua (Zoroark is a great choice to help you swiftly complete the game)
- Cinccino (Dig, a move able to be taught to this, can be very useful)
- Growlithe (if you are looking for something else, Darumaka outclasses this and is also attainable very early)
- Jolteon (Eevee) / Elekid (Electabuzz)
- Joltik (I would recommend the electric options above over this, however)
If you are looking for other suggestions for early Pokèmon without any very significant use:
- Tympole
- Scraggy
- Magnemite
Hope this helps!