It really depends on the card in question and what it's capable of. As Machamp the Champion said, much of our current metagame revolves around "Big Basics" type decks that employ Pokemon-EX and non-EX's with efficient attacks as their main force. The evolutions played in these sorts of decks, if any, (Garbodor, Aromatisse, Drigblim etc.) are either strictly in supporting roles or are fringe attackers used to respond to specific situations (techs) rather than main attackers. There are exceptions though, like Donphan, Pyroar, and even Eeveelutions, which have viability in the current format. Typically, decks that use evolutions as attackers seek to hinter the opponent (Donphan/ Pyroar) or hit for weakness on Pokemon-EX and grab two prizes while the opponent can only take one in return (Eeveelutions, Raichu, Drifblim, etc).
You will notice however, that all of the cards I've mentioned are Stage 1s. Personally I think it's unfortunate, but for the most part Stage 2s simply are not viable in the competitive scene at this time. This is partially due to the rotation of Tropical Beach, though some Stage 2 decks like Empoleon could still be competitive without it. The big killer though is Seismitoad, combined with the speed that "Big Basics" decks already have to knock out little basics before they can evolve. Seismitoad made it even harder, since it provides item lock, thus stopping the opponent from using Rare Candies. This means that Stage 2 decks simply don't have the speed to keep up with the metagame. There are exceptions, such as the Dusknoir / Mega Heracross deck that's gotten some fame lately, but the cards are very much stacked against them right now.
So in a nutshell, you're best with sticking to powerful Stage 1s with attacks that are geared toward taking out Pokemon-EX quickly or slowing their progress. Be advised that decks like Donphan and Eeveelutions are not simple, and require a lot of foresight and preparation to play effectively.