I'm going to assume this is ingame because normally a competitive question would have a team attached, but I guess so would an ingame team. Just not as necessary.
Dragonite: I wouldn't suggest it ingame because it takes a lot of training to evolve and usually hard to obtain (unless you're playing HGSS, but then it's at a weaker level [but with Extremespeed :D!]), but it is still really powerful and if you have its Multiscale ability, then great. Competitively, it doesn't have as much value in gen 4, but pre or post Sinnoh, it is "head of its class". In gens 1+2 it was considered one of the premier power threats, while 5th gen made it the best set up sweeper in the game with Multiscale+Dragon Dance.
Ingame: No, unless you do want to train and catch one.
Competitively: Yes, especially Gen 5.
Nidoking: This one is fun now that it hits Fairys well and can effectively handle steel types. It also gets BoltBeam which is a huge plus. Ingame, its easy to find and has a decent movepool between TMs. You might need to hold off on him however to make sure you get the right moves, but evolve him just in time to get Earth Power.
Competitively: Possibly more useful Gen 6, but is great in gen 5 UU.
Crobat: Crobat is a lot of fun. In X/Y, its hidden ability, Infiltrator, gets to go through substitutes which boosts its value, but it also gets Poison type STABs so it hurts Fairys. Ingame however, Zubats are easy to catch. Golbat might take a few levels to evolve, but you should go for it. It also could be the Pokemon to take you around town with Fly. :P
Ingame: Yes. It could be your Fly using Pokemon.
Competitively: Yes. However its weak to Stealth Rock so watch out. Best in the X/Y metas.