Superior Stamina pretty much always feels good, Majestic Leap is just stupid fun, and Healing Rite is a go-to lane survivability pickup; apart from that it really varies by hero. The default guides definitely leave a lot to be desired--I usually pick a highly rated community guide, as they do a nice job collecting good choices for the hero while offering enough variety to tailor a build to the match.

I don't know what I am doing so pretty much completely random.

Oh, the default builds are almost always non-optimal. They've been a good starting point though, but as I've gotten more familiar with the shop I've started modifying what I get.

Off the top of my head, intensifying magazine isn't in Wraiths default build, but is great on her. Also Haze. It does mean you want to fire your entire mag continuosly because if you let up you lose the damage bonus.

Also, the top right corner has a button for browsing community builds, you can pick them and swap them on the fly, to try different builds out and see what other people are putting on different heroes.

Some even have "branching" structures where you buy different items depending on how the match is going and who is on the enemy team.

Superior stamina, basic magazine, usually all of the cooldown and reach upgrades for better and more aoe. Other then that, I usually tailor it to the match depending on who I'm playing against and what they do.

*Also rapid bullets and maybe high velocity mag.

**Just found Pristine Emblem and Headhunter, but the game crashed before I could use them :(