Warning, most likely shitty formatting (first big post thingy, please be gentle), I'm sorry if your eyes bleed This also may be slightly more in-depth than intended, but what the hell

I've been playing Viktor for quite a bit now this season. One thing I really like about Viktor is that he has an extremely adaptive build path that can change game to game so there isn't exactly a set path and not a lot of items are garbage on him. The latest AP item changes released several patches ago reinforce the diversity in terms of items. Your items also depend on Runes/Mastery pages as well.

My normal setup for Viktor is MS quints, MP reds, scaling health yellows, and 15% CDR @ 18 and 21/0/9. - As I said earlier Viktor can be extremely flexible in build path so this isn't set in stone, I just like having the MS early on (helpful for dodging skill shots and in ties in nicely with your augmented Q) and capping out on CDR (most games) - I have variations of my Rune page (armor over scaling HP, MR over CDR) for specific matches I don't feel comfortable in or leans to the opponents lane more. (fuck Leblanc)

Deciding Build After Augmented E (basing off of my rune page set ups) - This primarily depends on how you are doing in the matchup up until that point. - If you are losing and took the route of no CDR (into an AP lane), I generally go Athenes (for the CDR lost out and the ability to farm out the lane without going oom) or Abyssal like you said - If going against Zed or talon GET THE ZHONYAS. Regardless of how well you are doing in the lane - If the lane is in your favor or even, you should go with the D-cap or Ludens approach (I only ever go Ludens if there isn't a spooky tank that is getting really tanky, really fast); I normally go Ludens after other items for end-game burst on carries - If you feel that you need more mobility quicker depending on their team get Augmented Q after Augmented E (if not then than after the items listed above)

Generally my final game is Sorcs/Lucidity, Hexcore, Rab, Void, Zhonyas, and then either Ludens/Athenes/Morello/Lichbane/Rylais (rarely)/Banshee's Veil (again rarely) - The last item slot is filled by either the lane matchup (listed above) or by need depended on the enemy team (if they have a shit load of cc Banshee's Veil is super nice) - The last slot for me is either the mana regen item (I love 40% CDR on this guy, 5.5~ sec E's are amazing) or Lichbane - I generally like Lichbane because it gives more MS and the popping power it has on squishies with just a Q>AA>E is insane when backed by Rab+Perfect Hexcore. - Rylais is an okay item in my mind as it gives a slow and more tankiness in general, but most of the time I never have gotten a great performance out of it - Finally Luden's as a last item: I know it is helpful for spikes in the early-mid game for Viktor, but personally I like getting the raw power from Rab earlier on so this is generally for the boost in AP and burst late game to 100-0 carries.

Hope this is in anyway helpful and if not let me know :P.... I got bored at work and this is what I decided to do.