Kettles

Electric Kettles We recommend that you look for a concealed element if you live in a hard water area.
Generally stainless steel kettles are less prone to limescaling than plastic, use a cadnit, Descale regularly to prolong the life of your electric kettle.
If boiling speed is important look for the highest wattage, cordless kettles are often rated higher.

Stovetop Kettles Look for a ground base or a thermic sandwich base if you are using a solid hob, Aga or Rayburn, especially if you leave the kettle on for long periods.
If you are using a gas hob a lightweight kettle will be fine although a heavy base kettle will last longer. Consider using a whistling kettle to prevent boiling dry.
For an induction hob you will need a base with a high iron (fe) content to complete the magnetic induction loop. Look for "suitable for induction".