Don are you an operator, or purely a collector? If you eventually intend to operate your trains, the key questions are: (1) How much space do you have; (2) What's your running style like? I.e., would you be satisfied with a couple of broad ovals just to see things move? A shelf-type switching railroad? Or a wide platform with a complex track arrangement? (3) What are your budget expectations (both initially, and per locomotive)? (4) How comfortable are you with technology, like command control, bluetooth, etc.?
There are no right or wrong answers. What I'm trying to get to is this... both Lionel and MTH made detailed scale models, AND traditionally-sized trains (usually with less detail and fewer features, sold for a lower price.) When you've answered the questions above, you can probably deduce whether you're a "scale"' guy, or a "traditional" O-gauger. I'm sure there are some people who run both. But you'll have fewer compromises and an easier time choosing; everything will be more cohesive if you put yourself squarely in one camp or another.
If you're a scale guy, you should generally look at MTH Premier, Lionel Vision (advanced features but pricey), JLC (more detail), and Legacy products. In their advertising copy, Lionel refers to these items "scale" or "Standard O."
If you're a traditional guy, you would probably be looking at RailKing, LionMaster, or LionChief Plus. Some of these items MIGHT happen to be scale-sized but the intent wasn't necessarily to create an accurate replica of a real train. Rather, it was to integrate well with existing traditional trains and operate on fairly sharp toy train curves as found on small-to-medium sized home layouts. RailKing "Imperial" trains are traditionally-sized but have a few extra add-on details.
The MTH DAP or Dealer Appreciation Program were a few uncatalogued paint schemes that were produced in smaller numbers and sold through authorized dealers. As Dave said above, the "Century Clubs" were purchasing plans. The first Century Club were traditionally-sized locos; the second batch were scale models. That's why I think it's important to figure out if you're a scale guy or a traditional guy. ou'll avoid two kinds of common disappointments: "This is too big for my curves!" and "Oh I thought this was scale!"
We might be able to give you more specific recommendations if you tell us more about your goals. Hope this helps!