I'd say an FL20.
As far as transducer- Dual beam transducers are a thing of the past for Vexilar. The Tri Beam 'ducers had an issue with cracking. Some people do not care for the Pro View 'ducer either. I had an old Dual Beam on my FL 20, and still use it on my FLX 28.
Vexilar does not offer a moving zoom range. Either bottom 6, or bottom 12. Some small difference between the FL 18 & 20 with their zoom functions. The FL 20 you go from hole to hole, you have to switch to full view and back to zoom on the 20. The 18 and 28 "bottom lock" will hop from hole to hole.
Target separation is great with any of them. I find the 28 to be superior, but more to do with the additional colors and finer line resolution.
IR- I think that most had 10 steps of IR, until the 28 (now 20 steps)
Popularity of Vexilar models- I'd say the FL18 was one of the most rugged units that had the best features. The FL20 was also a great unit with a few more features, and had the lager display. The new FLX 28 is nothing short of amazing to me. I got mine early last winter, and plan to spend many seasons with this one.