Honestly, best and the only one correct build by level 7 is getting one point in chain spike and one in marked for death, maxing strength sap as it's most versatile and useful spell he offers. What spell maxed next by level 11 has better synergy with sap+ulti combo and why?
The thing is chain spike does minor damage compared to 2-3 extra hits you can lay on enemy when ultimate is active and target is marked for death, but it goes from 1 to 2.5 sec immobilize and ye, it's not even a stun, that's why i think it's not worth to get more than 1 point in chain spike at all.
Do you know if the slow on his chain effect is the same at every level? I find that that is it's best utility. If so, then yes, i'd max it last.
But given the right laning partner + enemies I may max chain first for more burst/CC... I love killing lanes.
Slow is tapering therefore same for all levels and lasts for 5 sec, spike deals 140x2 physical damage at level 4. Marked for death always gives unitwalking and reveals targeted unit for 8 sec, boosting movement speed by 30% and attack speed by 40% at level 4.
I glanced at one of your replays and noticed you got steams. If that is your usual build I think mark is better to max first. If you alternate between ghosts and steams situationally, then it gets trickier. I would still give the edge to mark even with ghosts as it not only grants the MS for chasing but also a great bonus to AS (whenever I get ghosts I ALWAYS notice how slow I attack, and the AS from mark helps alleviate this). To put it in a different way, if you have a longer immobilize duration with a slow attack speed, does the increased duration really help you much?
i like to skip mark until 10, maxing sap first and chain second. I feel really mana starved if i get all 3 spells, and i feel like its usaully better he farms at the beginning anyway, so wasting gold on a chalice isnt good. Sometimes ill wait until 9 to get ult, other times i get it at 6, depends on lane and if im being focused in lane or not.