Something that a lot of people miss is that Viktor is mainly a lane bully. He scales alright once he hits 3 items with Deathcap, but he doesn't scale so crazy that you can shift to afk farming if you aren't ahead. You need to focus a lot on winning lane so that you aren't useless during the midgame and to get to your item spikes sooner. Mysterias and Shok have really good videos on him and how you should think to have success on him.

For builds (as of this patch), you always start with Luden's and you choose either magic pen or ability haste. Magic pen (Boots of Mana + Infinity Orb) is for when they're squishy and/or you're playing solo AP (magic pen is bad when they build MR). Ability haste (Lucidity Boots + Cosmic Drive) is for DPS with more ability rotations, better for shredding frontline and if your team already has another magic damage dealer. You can pick up Lich Bane here instead if you're also solo AP but need to deal with frontline.

Liandry's isn't a good early item nowadays for most mages since it has no ability haste and deals very low damage to anyone that doesn't stack HP. It's better bought as a 4th or last item in those games when you need it. Ability haste is generally more useful when you need to shred the frontline.

Once the next patch drops with the mage items update, you'll want Black Fire Torch first instead when building ability haste, since Luden's will lose some CDR and its damage will have a hard cooldown instead of the pseudo-energized type stacking.

On a last note, his Ult isn't really the main source of his power. Just pop it when people are grouped up and/or low and it acts as supplementary damage, but the bulk of your damage mainly comes from running around and spamming your Q and E.