The only talents that realistically matter, are the first 3. If you wanna snipe people, go Eye of Horus Ultimate, if you wanna empower a teammate, take Nano boost.
At lvl 13 I'd suggest movement speed, though others are good if they have slow+root or stun.
At lvl 16 Take Active reload if you want burst healing at certain windows, Sharpshooter to just heal if you're also confident with hitting it, and take Concentrated Doses if you're aggressive and actively hit enemy heroes.
At Level 20 upgrade whatever ultimate you took at 10.