Russian steel can be a good choice, but it takes an experimented player to make them work. I had a lot of success with A148 in previous GW3, but only because I could buy them for 13M$ new when the equivalent plane from Embraer was at 45M$. At similar purchase price, my A148 strategy would have been suicide. It worked only because I was the only one to buy them - this stalled prices. If many other players had joined, we'd have bankrupted together. No such risk with the Ejet, a sound plane that can be profitable even at high purchasing price.
If you stick with "classics", i.e. planes not too outdated and rather mainstream, it shall not kill you. In the modern era, turboprops up to 70 seats, Ejets(or the recently implemented Fairchil-Dornier 728 family that is nearly as good, and cheaper to buy) between 70-100 seats, anything Boeing or Airbus above, are families you can't make mistakes with. CRJs & ERJs are more tricky, because they lure you into long-range/low-capacity lines that are not that profitable, and I invested too much money in those. And they require anal-level price micromanagement to be profitable.
I don't know well the previous eras, though.
And be sure to stick to 2 main fleet groups, the 3rd fleet group being for fleet changes.