I don't think I've yet to have a wholly disappointing Pokemon in my team thus far. During my first playthrough I used :497::553::584::596::609::620:. It was more of a trial run where I picked my team based on looks. I went for the right nature and ability for all of them but did not bother with any proper EV training. The weakest link then was probably Vanilluxe, since it was only good against Dragon types, while Ground, Grass and Flying type can be covered by Galvantula and Serperior. And Ice type is naturally not a very defensive type so it was more of a struggle than usual. And ironically I struggled with Ghetsis because Vanilluxe was outsped and KOed by his Hydreigon, otherwise Ice Beam would have done its job.
On my second run though, oh boy, it was a like an all-star team (for in-game use), :497::508::558::563::601::628:. Before I started out a new game I decided on my team first, and then looked up Bulbapedia for their base stats, chose the moveset they should have using some of Smogon's information also EV trained all of them. Even Crustle, which was the weakest in terms of stats, did its job well. Did I mention that I was playing Set mode instead of Shift mode as well?
I see a lot of people posting Serperior here. I'm not sure what your Serperior uses, but mine's SpA/Spd EVed, given Giga Drain/Leech Seed/Reflect/Lightscreen and holding Light Clay. Pretty much the leader of my team, provides great team support and has little issues with getting KOed.
Some of the Pokemon you guys mentioned have piqued my interest though, I think I'll try using them when I have the chance to, unless it's one that I personally dislike of course. And it also helps if you post the moveset of the Pokemon you were disappointed with and its fellow team members, since it might be a result of bad overall strategy and coverage.