Sigilyph all the way. Like Swoobat it can learn fly, saving a teamslot. It's magic guard ability is brilliant and makes it very adaptable by negating indirect damage. It's also highly resistant to fighting and has decent defences, especially with cosmic power. It can also learn some decent moves and it's speed and sp.attack are excellent, outspeeding most things in game. Best of all, it ignores life orb recoil with magic guard, so if you get one it can make things go very well.
My one uses a modest (or timid) nature with psychic, fly, air slash and (move of your choice). It can also make great use of ice beam, which you get after after the E4, making it useful after beating them too.
It can be caught in the desert resort fairly early on, too, and magic guard is a regular ability, rather than a dream world one. Definitely recommend Sigilyph.
Sp. attack: 103
They appear fairly often in the desert resort. The magic guard ones can be spotted as they take no sandstorm damage. Poisonig them won't reduce their health but will still make them easier to catch, another reason why I love that ability. Wonder skin is inferior, but workable. I'd get magic guard for the best results.