Best plant commercial plant liquid fertilizer?

I'm using Kemps brand but I haven't had it long enough. I'll let you know how it goes.

Many folks on this board are fond of the Seachem line of products. Flourish and Flourish Excel are names you see alot while things like KNO3 and KH2PO4 you can buy buy the pound online and have enough to last a long time. I am using Flourish and Flourish Excel and dosing couldn't be easier unless it jumped into the tank by itself, and my plants grow wonderfully well.

I have had very good luck with the Seachem line.

If the nutrients you're adding are limiting the growth/health of your plants, then yes, they will help your plants grow faster or more heathy. If not, you're wasting money and encouraging algae.
Depending on what you have (light, CO2 or not, fish load) you may need more of some nutrients/less of others. This is where test kits come into play and analizing labels.
If you're just looking to add something w/o the hassle, go with what you're using now. If things seem better, stick with it. If not, try something else.
You're really taking a crap-shoot w/o testing the water though, b/c you'll never know if it was the brand of fertilizer or over/under dosing if you don't get the results you want.
HTH