Such as picking up a Riolu at the start of the game, are there any other Pokémon that can critically make game play easier?
(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:
If you are looking for other suggestions for early Pokèmon without any very significant use: - Tympole - Scraggy - Magnemite
Hope this helps!
Once you can get it, cobalion. Great false swiper and overall fighter. And I suggest maybe azzumail for hm slaves. And mareep. False swipe is in reversal mountin. Theres a scientist that will give it to you once you see or catch all the Pokemon (not including heatran) in reversal mountin. Once you have them all, talk to the scientist and you got it.