They don't have as many albums as In Flames. As far as output, they are more akin to the older thrash bands that put out a few records, broke up and then recently put out new stuff.
I haven't heard the new stuff.
As to where to start, Slaughter of the Soul, without question. And people didn't really start all of a sudden going nuts for ATG, heh. Slaughter is arguably one of the most important metal albums of the 90s. Not to oversell it, and it might not sound like a big deal now, but it was then. Even if you don't end up liking it, it was a pretty big deal.
I also would not say In Flames sold out to flavor of the month trends either, personally. Their sound evolved, no question. I like the later stuff. Not like a die hard fan, but I think it's good.