Haunter

In spite of Haunter's extreme frailty, it manages to carve itself a niche the most offensive Ghost-type in the tier due to its nice STABs, Speed, and Special Attack. Because of its Speed and movepool, Haunter also functions as a really good annoyer and revenge killer. It also has really nice defensive typing and ability, granting Haunter three immunities and some nice resistances, allowing switch-in opportunities in spite of its frailty. You should try to avoid switching Haunter into resisted hits, however, as even resisted hits do quite a bit of damage to it. As such, Haunter is not the greatest spinblocker, because switching in on the wrong move spells the end for it. However, Haunter is a very useful Pokemon, capable of either disrupting the opponents strategy or sweeping the opponents team.