Always nice to cover as many bases as possible while also making sure your team is difficult to counter... Can't think of a full 6-poké team but can make a few suggestions:
1 Legendary has to be Zapados. His two types' biggest weaknesses (ground vs electric, electric vs flying) are covered, and he packs a huge punch with Thunder and... eh... <insert best flying attack here>.
As for some others, Dragonite's always a nice bet. Only weak against Dragon, and has very high defense for combatting physical attacks. Can learn some pretty sweet attacks, and a well-timed Hyper Beam can win you the game.
Venusaur should be overlooked, grass isn't really that useful an element in competitive gameplay, and he's just too slow to be really useful.
Gengar is one I normally include in tournaments, simply because he can't be touched by physical attacks. The addition of poison attacks over Haunter make him more viable. Poison can be a real pain. Land a Toxic and the other poké's going to either get hurt pretty bad, or the other player's gonna burn a turn swapping, or healing (if you can use items). Plus, a level 100 Night Shade does quite a nice chunk of damage regardless of what element the other poké is. Pretty useful if you come up against something with rediculous defense (maybe your opponent is crazy and uses a Snorlax with maxxed out defense and special defense... or a Chansey that likes healing itself).
Great fire pokémon are rare outside Charizard and Moltres, and are situational at best, there's not much they're great against that you can't cover somewhere else. I'd choose either Rapidash or Flareon because of their speed. Rapidash would probably get my vote since Stomp can cause the opponent to flinch while still being very powerful. At least, I think it's stomp.
Alakazam is a very strong pokémon to have in your party too, especially if you play him as a disruptor - that is, constantly keeping sleep or confusion stacked up on your opponent. Hurts quite a lot too.
If you really need a fighting type, then pray you've taken Hitmonchan. His elemental attacks are very useful for delivering a lot of damage to something that gets thrown out after you defeat a pokémon, or it gets switched out.
Basically, in tournament play you're looking to do a number of things. Minimise switching, they're wasted turns and you'll take damage that's mostly unneccesary. Also, I've found personally that preventing damage is more effective than causing it with the team I normally play in Pearl/Diamond tournaments, and the strategy works well in RBY too.
As far as moves go, do a bit of research. See what your pokémon of choice can use, and try to give them something that's an element other than their base one. It gives a little bit of flexibility, and makes it easier to have a counter for every element in a party of 6.
Good luck and let us know what team you end up picking, and how you do.
Shout if you want more info on anything
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