What is the best 18'' light for plants?

You should avoid actinic and 50/50 bulbs that are meant for reef tanks, but other than that, all bulbs that with fit your fixture will be more or less the same when it comes to growing plants. Those that are labeled as "plant growing bulbs" actually aren't any different. They still run on the outdated myth that color temperature is an important factor in plant growth - most "plant bulbs" are around 5000k to 6700k for this reason. The fact is that what matters to growing plants is the level of PAR (photosynthetically available radiation) a bulb can create, which does not correlate with the color temperature. There isn't any way to know how much PAR each brand of bulb can create without using a PAR meter and testing it yourself, but they're more or less the same and are all probably within a 5% margin of each other.