I got into them around the time of their third album, Betty; so for me, that will always be their defining record against which all others shall be judged. But for most, Helmet's definitive masterwork is the previous 1992 release Meantime, for which they happen to be embarking on a 20th anniversary tour, for which I happen to have purchased tickets, which is largely why they have popped back to the front of my musical consciousness again.
Helmet were, and are, awesome. They are of an age that many modern bands are now citing them as a huge influence, and many have covered their songs - from Pig Destroyer to Deftones.
They are a seminal hardcore band, and you should most definitely give a massive shit about them.