I have to disagree with everyone.
70-140 seat: BAE-146/Avro RJ
Pretty cheap, easy to get on the used market, great size, lasts a decent amount of time.
140-200 seat: McDonnell-Douglas MD-81/MD-83 and 737-800
Mad-Dog 80s: Cheap, easy to get used, range is bearable, but can match my airline's needs easily. On longer flights, I use 737-800s equipped with winglets.
200-260 Seat: Airbus A321-200
I very much like this plane when needing to fill passengers on a plane. Easily suites the needs of my airline, but I usually have to get them new.
260-350 Seat: Boeing 767-300ER
I got this plane when working at another airline. I don't usually buy planes of this size, as I rather operate more flights in smaller planes, but was defiantly a well rounded plane.
350+ Seat: Boeing 767-400ER, Airbus A340-500, and 747-400ER
The 767-400ER is a decent plane, uncommon, but can do decently long routes. The A340-500 is my choice of plane after 2002 for trans-pacific routes that are incredibly long. The 747-400ER packs passengers, not much can be said, but I love it.