the "best" slide in out there is anything with a manufacturer that will stand behind its product, and a distributor conveniently located for you.
other than that, as a multi-line distributor, i can tell you that the slide-in machines we have received the best feedback on are HM's
Boxxer 421, and pretty much any Blueline machine. Despite the large number of units we have sold, the 421 has been an extremely reliable machine. One of our local customers has one of the prototype units, and aside from a couple modifications that were implemented during the prototype run (such as improved baffles in the waste tank), he has had almost no problems with it, and is now looking at buying his 3rd
Boxxer this spring. For other Boxxers we have sold, other than the little stuff that wears out here and there on any machine, the only problems we have had were all operator error (for instance, people NEVER changing their oil, and losing a motor as a result).
We have only been carrying BL for about a year, year-and-a-half now, but we have sold quite a few of their machines, and gotten raving reviews from our customers, and have had VERY few problems. But to reiterate: ANY manufacturer with decent distributor locations who will stand behind their product is worth looking at.