A dull knife is not only useless, it’s more dangerous because you will make up for that lack of a sharp edge with more pressure. That means, when your knife slips, you cut yourself more deeply. I have spent enough time in the ER reflecting on this to become somewhat religious about sharpening my knives.
In an ideal world there would be one sharpener to rule them all and I would just link to it here. Unfortunately, that’s not how it works. Different blade materials must be sharpened differently, in particular many modern stainless steels are too hard to effectively be sharpened by traditional water stones. (Water stones are great for carbon steel, though. I love these Shapton stones.)
Strange as it sounds, my most-used sharpener is Benchmade’s Guided Field Sharpener. Technically it’s meant for smaller blades, like pocket knives, but I use it on kitchen knives every day and it works great. If you want something bigger, I suggest diamond stones like DMT’s diamond whetstone set ($78 at Amazon).
For more sharpening options and some tips for how to put the best edge on your new knives, check out this guide, how to sharpen a knife the right way, from Epicurious.