Your best bet is to go with a stainless steel kettle. While these are more expensive than the other options, a stainless steel kettle will last any brewer a lifetime. We suggest purchasing one of at least a 7 gallon capacity, that way if you decide to move up to all-grain brewing, it will be large enough to boil an entire 5 gallon batch.
How about those enameled kettles? These are fine, but make sure that there is not a chip anywhere in the enamel. If the enamel becomes chipped, this will cause off-flavors and haze.
A couple of definite no-no's when considering a brew kettle:
Do NOT use a cast iron kettle. The acidity of the wort will leach iron into your wort causing off-flavors and haze.
Most brewers would advise against an aluminum pot; as aluminum may dissolve in the acidic wort, causing off-flavors.
Some tips on using larger kettles
Start with 10% more wort than the intended batch size, to account for ~10% boil off over the course of an hour
Choose a kettle with 20% extra volume headspace - largely to defend against boilovers