To be honest the safest build is generally the one you feel the most comfortable playing yourself.
Out of my beginner builds I would say that both the vitality casters and the aura Paladin are really safe and easy to play. The Spellbreaker, being a dual wield melee build, can be a little harder to pilot.
Generally any kind of 2h or shield soldier is also very safe, I can recommend my deathknight, warder and warlord. For leveling DK, best is to follow the beginner Blade Arc guide by @Stupid_Dragon [1.1.9.0] Beginner's Apex Death Knight, for Warder and Warlord there is probably some leveling guide aswell or just level with forcewave like my “how to level a soldier” video shows. Warder can also be leveled nicely with savagery and a troll bonecrusher or voldrak’s crusher and cadence. Warlord can be leveled as almost anything, it’s a really strong class, my favorite way to level it is probably Blitz&RF right now.
Generally when it doubt, forcewave out.