I'll be the odd one out and say that I actually like the Lionel 21" cars quite a bit. They are more or less ABS copies of the K-Line tooling and generic, but for the price they are a decent value. I'm up to 23 of the Lionel cars between Amtrak phase I, phase III, and ATSF. They fill out my passenger trainsets nicely. I consider them "background" cars that allow the road specific ones to stand out. On my ATSF train, I mix them with GGD heavyweights which makes for a very nice secondary train behind my E8s, E6s, and E1s.
On the Heritage Amtrak cars, I need to cut some skirts off, add some underbody detail, and decorate the interiors. However, at the price I have been getting them for, that is not a huge burden. Lionel got me 2/3 of the way to a correct 1988 Broadway Limited so no complaints from me.
I still enjoy the K-Line aluminum cars and have probably 40 plus in multiple roads. When the price is right in the roads I collect, I don't hesitate to pick them up. Great cars for their time and they still hold their own in many ways. I have K-Line in NYC, PRR, Amtrak, SP, and a lone ATSF 21" business obs.
In plastic, the Atlas CZ cars are the gold standard for streamlined cars and the Alaska ones are very nice and prototypical. In general, the CZ cars were long lived and many still exist in private ownership. For aluminum you really can't do better than GGD for fidelity to the prototype. We haven't done Alaska yet, but it would be interesting to see what tooling has been done by GGD that matches the cars that Alaska purchased.
Alaska railroad has a really interesting history that is certainly worth learning more about and they still provide a necessary service to a remote part of the US. Great stuff!