Class 47:
Lima - Cheap single moulded plastic though motors are generally quite poor due to age.
Hornby RailRoad - Cheap single moulded plastic Lima shell on Hornby driving mechanism.
Bachmann - Not sure but better than the Honrby one.
Heljan - Not sure but generally heljan models are usually V. Good.
ViTrains - Reasonably priced though most details have to be added separately and not made any more.
Class 86
Hornby - Old moulding so detail isn't brilliant, No lighting.
Heljan - Not sure but has lighting. One would assume it is better than the Hornby one due to newer moulding.
Class 90
Hornby - Really old model (so low detail) still being made and being sold over priced.
Bachmann have announced they are working on.
The Hornby Railroad 47s are actually the old Hornby ones and are really poor models (and expensive), The Lima body/Hornby chassis version has only been released in main range train sets and packs.
The third best 47 is generally recognised to be the Heljan one, although I don't know how many are being made now, they were to wide which earned them the nickname "tubby duff". Some have been know to suffer from "expanding chassis" which isn't great, but by now I'd have thought they'd either have expanded or they aren't affected.
Strictly speaking I believe the ViTrains 47 is the best, but it has a large number of fiddly extra parts that you need to add on (Rail magazine's "expert" reported that he took 90 minutes putting them all on) and they aren't being made at the moment.
The Bachmann 47 is a damn good model (it was thought very little could improve it) and Bachmann haven't stopped making them, but with prices rising all the time, the secondhand market might be the best option here too.
The Hornby 86 is crude (and underpowered) by today's standards, but there are people who still prefer it over the Heljan example, partly because the Heljan Pantograph is equally crude and sits too high on the roof. Some have also commented that the cab and roof profiles on the Heljan example are wrong, I'm not convinced about the cab either way to be honest, but the Hornby one is more plentiful in number and livery option. Early examples of the Hornby 86 do have really crude Pantographs, so watch out for them.
Hornby never did a RES livery 90. There was one in "Express Parcels" (a livery the 90 never carried) which isn't so very different, but it might be worth waiting for the Bachmann 90 to hit the market, firstly to see how good it is, and secondly because the price on Hornby 90s might fall.