i might suggest tyranitar... look him up and u will soon see why... --- Fat? I ain't fat. So what's this you ask? That's my uhhh... giant one pack. Hmm? most people have 6? I know, I drank the other 5!
it's difficult to obtain Tyranitar in Emerald,so my bet's also on Houndoom - he's speedy and packs STAB flamethrower(or Fire Blast) along with Crunch. Will-o-wisp is a must move for those pokes you can outspeed but cannot beat - breed with vulpix/ninetales for it. Shadow Ball could be nice move too - it doesn't get STAB,but covers Psychic and Ghost,and can hit Fighting without being "not very effective" - of course you'll lose the chance to hit Normal pokes with it..
I like Houndoom, but the defenses just suck - one good hit will bring him down,so you should einter rely on putting a dent on the opponent with Fire or Dark STAB moves,or you burn it,which will be good against physical sweepers (beware of Guts though)
IMO Sableye's not a good pokemon,even though it doesn't have weaknesses - it's rather weak, but does have a good movepool to use..
If you use the pokemon in-game, maybe every dark pokemon will work - but in competitive play,Houndoom will be your only chance of doing something - and even then it's not so good - however, if you can get your hands on a Tyranitar in Emerald,it'll own - Sandstream is very good Ability,and the poke's a killer. Too bad I don't have anybody to trade with - I'd love to have a Tyranitar in my team.
P.S. - Shiftry !!!!!!! One hell of an annoyer,if you give him Chesto berry and teach Dig/Double Team/Leech Seed/Rest . Another good move for it is Explosion. I can think of Glalie too,although it's not very fast,but has Explosion too,Hail could hurt some pokes,Ice Beam is always an useful move,and both Ice Beam and Crunch receive STAB.
At least all of these pokemon are useful in Battle Frontier..