There are airlines which believe that fleet commonality is very important and try to have one aircraft family as the backbone of their fleet. There are also airlines that due to their size or various missions, have acquired various different types.
IMO, CO has a great fleet.
B733/B735/B73G/B738/B739/B753-- Domestic and North American ops
B752/ /B764 -High capacity domestic and international ops
B762/B764/B772- Long-haul international ops
Engines: GE-CFM and RR(757)
The beauty of CO's fleet is that most of their aircraft can be used for various ops effectively. B739 could fly short transatlantic ops, B752 and B764 can do a great job domestically and internationally. My point is great fleet flexibility with the best quality aircraft. (Massive Boeing bias)
Air Canada on the other hand
CRJ/E170/E190/A319/A320/A321/B762- Domestic, US and short range international
B762/B763/A330/A340 - high capacity domestic and long haul flights
Engines: GE-CFM, RR, PW
AC Future fleet:
CRJ/E170/E190/A32x- domestic, US, some International
B772LR/B773ER/B788/B789- International long haul
AC too has great fleet flexibility, but they are also forced to take B763/A330/A340 out of international ops for high capacity domestic flights like YYZ-YVR.
Crew training, maintenance, spare parts, engines, interior commonality are all advantages of having a streamlined fleet.
So what airline do you think as a "strong" fleet and why?