Definately Broodlings
Yeah, Broodlings are great for kicking the shit out of closely packed tanks, but you're forgetting what air units can replace the effectiveness of broodlings... guardians. Here's a unit that's going to take a little bit more to kill, won't die after one hit, and tanks can't touch. Yeah, they could get wasted by wraiths, but that's what Ensnare is good for if you don't have an overlord. Additionally, you can easily cover air with some hydras out of range of the tanks with a defiler to add some darkswarm, or some friendly Mutas and scourges.
Granted you can kill one tank for the cost of nothing once you research 'Spawn Broodling' but think of how long it takes to get that energy... let's say there's a group of 20 tanks all together, blocking an entrance for 3v3 0 clutter or something. It takes 150 energy to spawn one set of 2 broodlings. Now if you consider the fact that this player obviously has more than 2 factories to produce all these tanks, than you'll know that they'll be quickly replaced. If you think attacking right after you do this is going to work, you might be sadly mistaken. Splash damage would take out what... 4 or 5 adjacent tanks, tops. Plus the player (if intelligent enough) would have air support, entailing either mass wraiths, or some BCs as well. That right there spells diasaster for the mishappen Zerg player. Spawning Broodlings is something better left for ladder maps where every mineral counts, and you can use that ability to make the player constantly replace expensive units =)
