Stealth Rock. Spikes can't reach its full potential unless it's used thrice (and if you're the one setting up the entry hazards, then using it thrice can be annoying). Stealth Rock can potentially do only a little damage, sure, but aside from that it can also do a whole lot (even more than Spikes when there's a x4 weakness), a lot of good Pokemon are weak to it, and most importantly I believe that there are almost no Pokemon that are immune to it, unlike Spikes where lots of Pokemon are immune to it.