Previously, I used the MD-90-30s (and before that the MD-80s) quite successfully. They have a few more seats than aircraft of comparable cost. However, recently, I have used the A319s as my main fleet (I tried, and didn't much like the A318s), with A320s and a few A321s for larger cities. That worked quite nicely too. I haven't used 737s much, but I don't see what would be the problem based on the stats. Really, with any of those aircraft, you will make good profits as long as you have a reasonably big base. The 100 to 200 seat planes are probably ideal for most airlines. If you are in a smaller city, you may naturally want to consider using smaller aircraft...