I just hit rank 5 mastery with Rengar this preseason, and here's how i build him:
AD, AS, Armor, and CDR runes
12-18-0 masteries (thunderlords)(can also go 18-12-0 with fervor, but not quite as bursty)
Start machete + 3 pots, and after i complete
, the items i go for are:
(even better for Rengar now, and active so useful during ult)
(Tank items are situational, but this is the most optimal one)
(Rengar needs every stat)
/
(mobis if you're even, CD boots if you're fed so you can ult more often)
Other good items:
(not as good to rush anymore, but still very strong once completed)
(if you're falling behind a tank-heavy comp)
/
(for MR)
If you find yourself falling behind, it's not a bad idea to pick up 2 or 3 tank items (typical stuff). I haven't tried rapid fire cannon, so i can't judge it's effectiveness.
If you want other general playtips, just let me know :)