I would recomend the largest kettle your burner/stand can support (within reason). You may be doing 5 gallon batches now but what if you decide you want to do 10. If you go with a 15 or 20 gal kettle now you will be able to grow into it if you want.
For 5 gallon you need at least 7 gallon kettle but 9 would be better.
I use all Stout Tanks kettles http://www.conical-fermenter.com
They are quality and have served me well. He can customize to your hearts desire so if you don't like what he has in stock just shoot him a message with what you want.
If you want to spend less, bayou classic is fine too. I used to use a 15 gallon that I picked up for about $100
Conical : bohemian pils
Keg 1: Stout Envy Porter__Keg 2: Pale Ale
Keg 3: Russian Imperial Stout__Keg 4: Pale Ale
Keg 5: AIR 6: Edworts Apflewien
My HERMS rig