They all have different strengths, whether it be the low exp pen of a human (which makes learning assassinate easier, imho, because the people you want to stalk will still be learning in areas that it is easy to stalk them in, quite often), the permaflight and dodge skill of an arial (at the expense of some vulns), or the brawn of a felar.

Drow have pretty good dex and there are a lot of times when permasneak is nice. But then a lot of enemies will be using mithril on you.

Halfies are sort of all-rounders but nothing special. imho.