I like most everything Triumph did, minus the last album that Rik Emmet wasn't on, but if you want to grab what I consider their best albums, get Just A Game, Progressions of Power, and Allied Forces. They really started hitting their stride on Just A Game, Progressions Of Power is arguably their heaviest album, and Allied Forces is a classic. Never Surrender and Thunder Seven are good too, but not to the level of the three I recommended. Their earlier albums are a bit unfocused, halfway between progressive rock and straight up heavy metal, but I have a soft spot for "Blinding Light Show/Moonchild" off their first album. First heard that on the American issue of Rock And Roll Machine, which was actually their self titled debut and the Canadian Rock And Roll Machine's best songs stuck on one album, and promptly fell in love with it.
If I had to recommend just one-Allied Forces. That album's a MONSTER.