I have used quite a few, it depends on your needs. Some are so expensive that they are only suitable for large companies. I would suggest shopping around BUT please please for your own sake do not even think of going with the Geneva system by Navitaire. I've had the bad fortune to work with it in 2 different companies and it is complete garbage. The most user unfriendly out there. Lisline is quite good and easy to use..
happy hunting
SJ