Assuming you have enough IP for at least two full pages of runes (one AD and one AP) then i would recommend Darius first. Be warned that darius is considered OP right now and may be subject to nerfs in the not too far future. Zed on the other hand is often touted as an assassin, but his assassination pattern is extreemly counterable. He is better served as a split pusher, which takes a good amount of map awareness and warding that is harder for most people to understand than the mechanical skill required to execute the fast and accurate combos that people say are hard to perform.
Others i would recommend trying out are
top: sion, lulu, pantheon, garen
jungle: amumu, pantheon, warwick
mid: twisted fate, lulu, katarina
adc: caitlyn, ashe, corki
support: alistar, lulu, nami
That way you have a decent amount of role diversity and you can learn a lot from each of these about various playstyles. You almost certainly wont enjoy all of these champions but it will help you determine which playstyles you do enjoy.
But save up for full tier3 rune pages first if you haven't yet.