It really depends on your play style, local environment, what you want to do in the game (attack a lot vs. try to defend gyms as long as possible vs. mostly just collect all of the pokemon out there), and so forth. Personally, I've powered up at least one of each of the best attacking/defending mons: my list includes Dragonite, Vaporeon, Arcanine, and Exeggutor (and will also include Lapras and Snorlax if I can ever get my hands on either of them!).
I also think it's generally a waste to power up mons beyond level 30 (with some exceptions); the CP gain for raising mons above 30 is outweighed by the massive stardust commitment.
