Babydreaming You won't battle with Tangela or Pineco, so they're not worth powering up, and Misdreavus evolves into Mismagius in Gen IV so you don't want to power it up until Gen IV is released.
Charmeleon turns into Charizard, which is a really good fire Pokemon, so it is worth powering it up as you'll battle it against grass Pokemon.
Also because Rhydon is so good, you want it to have high IVs, so at least 85%, but anything higher is better. So I wouldn't evolve a Rhyhorn into a Rhydon until you have a high IV one.
The same applies to evolving as powering up: don't evolve the rarer Pokemon unless you have a high IV/high CP one, as otherwise it's a waste of candies.
When picking Pokemon to battle it does get very complex, as you need to take into account the moves it gets, the CP, the IV, its type and what it's good against etc.
Since you're not sure what to battle with, I would focus on getting the best of the following Pokemon:
• Gyarados (Water/Flying type, good against Fire, Steel, Water, Bug, Fighting and Ground)
• Tyrannitar (Dark/Rock type, good against Flying, Fire, Normal, Poison, Dark, Ghost and Psychic)
• Golem (Ground/Rock type, good against Flying, Fire, Normal, Poison, Electric and Rock)
• Rhydon (Ground/Rock type, good against Electric, Poison, Rock, Flying, Fire and Normal)
• Machamp (Fighting type, good against Bug, Dark and Rock. Very useful for taking down Blissey and Snorlax)
• Dragonite (Dragon/Flying type, good against Fire, Grass, Water, Bug, Fighting and Ground)
• Ampharos (Electric type, good against Electric, Flying and Steel)
Those are the really the only ones you want to focus on powering up. There are others worth powering up (Vaporeon, Eggexcutor, Blissey, Snorlax etc.) but those listed above are the main ones. If you have other Pokemon outside of those I would ask/check/google before powering up.
Hope this isn't too confusing! 