pupperoni

If we're talking about modern planes (after 1980),

50-100 seats: I really like the Avro RJ/BaE 146, it can take off from pretty much anywhere, is faster than turboprops, has decent fuel burn per seat, and has enough range to connect the US east coast, Europe, or most of China

100-200 seats: A320-200s are hard to beat for efficiency, they're really well-rounded. Often the most common plane in my fleets. The A319LR is also amazing for when I need to connect small transatlantic airports, since it's got such a long range and can be used profitably for both short and long haul.

200-300 seats: As a cheap starter plane, the Tu-214 is unbeatable. It costs less than 1/3rd of an A321 but has nearly as many seats (210) and the fuel efficiency with winglets isn't bad either. For the upper end of this size class, the 757-300 is amazing. It has 295 seats, is cheaper to buy than an A320 or 737, and can fly transatlantic, China to Europe, Dubai to almost anywhere, or even just connect local cities.

300-500 seats: I like having a mixed fleet of A330-300s and A340-300s. The 340s are more efficient and can fly further, but need a longer runway and have a longer turn time. The 330s fill the gaps where the A340s can't fly. Later in the game, 787-8s and A350-900s are always amazing. I think the A350-900 might have the best fuel burn to passenger ratio in the game, not to mention its incredibly good range and good runway performance.

500+ seats: I usually don't have a ton of planes in this size category, but I always have some 777-300s because they're very good long-range large-capacity planes. Their only downside is they need a really long runway. When the 747-8 comes out I will usually pick up some of those, I like them more than the A380.