Archer build, 15/24

[Nehvar]

That'll get you only 147 archery @30 so it's going to depend on what bow you're using.  Also you'll have a rough time evading melee types so you better have some parrying ability.

[BattalGazi]

In means of parrying, you are talking about atlethics am I right? To tell the truth, I just want to be able to pin down players with an alt. char. ,so no heirloomed weapons at all.What would be a choice for these circumstances?

[Nehvar]

Yes, fifteen agility will mean that the majority of the melee types out there will outrun you so you better be able to parry(block) decently.  I wouldn't go below eighteen agility, myself.

21/18, 18/21 or 15/24

I only have the one first-generation archer with an 21/18 build so I'm not really qualified to tell you which of the three is best.  I can say, though, that even with eighteen agility is still have problems with being run down by certain melee builds.  (Usually longsword or awlpike users it seems.  Those bastards get priority after horses.)