what's the best build for rogues/stalkers?

[Irxallis]

Personally, I think dragon killers are essentially useless.

I disagree with you.

1) If you happen to go to Abyss party, you want a Double Dragoon Dragon Killer - weapon switching would require too much attention (we've been there and done that >.>) and you can do nothing sensible against golden acidus (VVVS damascus ftw?). Dragon Killer is a good weapon for that location. This weapon combined with Aspersio and plagiarised trifecta allowed T to down a Hydro (so we managed to outdamage those 9999 heals, though it wasn't the fastest Hydro killing ^^;). Not to mention it is good to pierce the OMGinsane defence of Dragon Eggs.

2) If you go to Nidhoggur1, Dracos are Earth1 only. Dragon Killer way outdamages VVS fire damascus. You could even get Fire Endow on this weapon, which makes it useful against anything there except Rata and Ancient Tree - and most enemies you meet on lv1 are Dracos and Draco Eggs. You can leave everything else to other partymates and become The Draco Killer...
...assuming you have a Soul Link, of course <frustration>.

Granted, I agree that dragon killers may not be cost-efficient in a soloing situation but I definitely want that weapon in several spots. It is like an Ice Pick - +7 directed mes is superior to it in most PvM spots but there are some spots it just shines.

Dragon Killer being useless would be like saying that weeder knife is useless - and then you go meet an Ancient Tree or get a fire endow and massacre mavkas. A situational weapon. Not something I would invest in before getting +6-7 VVS ele / VVVS ranker damasci, but something T wants in her collection.