Any of these Pokemon will work....
Slugma
Magcargo
Magby
Magmar
Camarupt
In Emerald, the abilities Flame Body and Magma Armor will hatch your eggs twice as fast. Other abilities in the game do other stuff as well. Coolest being...
Compound Eyes (Butterfree/Nincada/???)- Doubles chance of wild Pokemon holding Item if in your first slot
Synchronize(Ralts/Abra/Espy/Umbry/???)- Wild encounters will 50% be the same nature if in your first slot.
Keen Eye/Pressure/White Smoke- less wild encounters if in your party
Hustle/Vital Spirit- more wild encounters if in your party
Sticky Hold/Suction Cups- Makes Fiching Easier (handy for the Feebas hunting...)
Sand Veil- Less encounters with wild, in a sandstorm if in party
Magnet Pull- Makes Steel type Pokemon encoutners more frequent if in Party
Static- Makes Electric Type Pokemon Encounters more Common if in party (Use combo with leading off with Nincada, have Voltorb and Magnemite in party to increase odds at getting rare Metal Coats since you will basically encounter only Magnamite/Magnaton in New Mauville)
Cute Charm- Makes encounters with Pokemon of opposite natures more common if in party
semi-completely useless Abillities
Hyper Cutter- Bigger radius when you use Cut HM
Lightning Rod- Increases number of phone calls
Swarm- Increases number of wild calls from Pokemon in the background
PS: while breeding, if Ditto or female is holding an Ever Stone, it will give you a 50% chance egg will hatch same nature as the Ditto or female.