Small Regional Jets
F-28 ($24000K), F-70 ($38000K), Fokker 100 ($57000K), DC-9-40 ($47000K), DC-9-50 ($53000K), BAe146-100 ($36000K), BAe146-200 ($44000K), BAe146-300 ($51000K), A320, 707, 717, 727, 737,and 757.
They all cover the first half of the game while the F-70 and the F-100 are available until the mid 90s. The 737 is available since 1968.
CRJ -100, -200, -700 ($22000K-$37000K, the latter often being converted to a 22-seat business-only aircraft)
Small regional aircrafts are usually great for opening routes that are easy to keep as a monopoly and/or from a large city to little cities with a tourism around 120-180 and low business ratings.
Some find that places like the Canary Islands, Key West, the Eastern European cities, the little Indonesian cities around Kuala Lumpur, and Singapore, the small-but-well-rated Korean cities, some of the Appalachian cities in the US, outliers like Nuuk, or even central Asian cities like Yerevan don't get much attention, but there is a nice little 1,500K to 2,000K per turn to be made of a regional jet route to those places. Also since there is few competition you can usually set the rate at about 1.4 - 1.6 and let it cover most of your airline's expense if used in large populations and with the right method.
When your passengers have no choice, satisfaction may not affect the occupancy a lot.
You can usually pick up one of these planes with little money after buying big birds for your main routes
Suppose you have a Concorde and a CRJ-200.
The Concorde can make an average of 15,000K every month and throughout 20 years. The cost of buying one Concorde is 399,000K.
While theCRJ can make 1,800K every month. The cost of it is 24,000K.
Excluding all expenses, a Concorde can make (15000/399000)*100% of profit.
That is about 3.759% of its value. At the same time, 0.0041% of its value vanish every month due to deprecation. (That is about 1635.9 K per month)
While your CRJ-200 can make 1800K per month; That is 7.5% of its value and whereas every month you just lose 98K due to depreciation.
Therefore it is advisable that long range and routes with huge demands to be operated by large planes and small routes to be operated by small planes.
Concorde isn't being operated in major routes between cities with 300 tourism and 200 business ratings, so it is hard to make the price tag in 1.3 and getting 100% occupancy. Any competition can easily destroy any of your routes that has the Concorde. And if the ratings is not 150+, you can never make more than 8000k in high seasons.