I'll concentrate on payware (since freeware is free, if you don't like it it hasn't cost you anything!)
US
Ohio Steel (Dover Division) is fantastic, I love it. And you have the added bonus that if you buy it from Steam you get the highnose NS SD40-2 thrown in as well!
Colton and Northern is a fictional route - whilst I usually much prefer real routes this one feels right.
The Horseshoe curve is lovely - it reminds me of happier times.
Portland Terminal is a lot of fun too.
The NEC runs too slowly on my PC, personally it's a disappointment only for this reason.
The G-Trax steamers are great, I prefer the SP&S Northerns and the NKP Berkshires to the SP GS-4 but that's no reflection on the quality of the product. Super freight wagons to match can be bought very reasonably from Golden Age Of Railroading - I particularly like their USRA 40' boxcar packs (currently being revamped) - you get a wonderful selection of different companies' boxcars for only a couple of pounds/dollars. Fantastic!
The Challenger and Big Boy packs are very nice indeed and I think essential purchases.
The GP9 pack is great, you get a lot of different liveries for your money, as you do with the SW1500 pack.
The SD70 packs are nice, model is good but a tad expensive for the few livery variations you get (particularly true of pack 1)
I like the warbonnet SD40s too.
UK
Where to start? The UK is the country best served by Railworks.
I was born near Reading (the English one) and my family moved to just outside Glasgow when I was a child, so I am also very familiar with the Edinburgh-Glasgow and WCML(N) routes in real life. I can vouch for how accurate the addons are; I don't just know where my old haunts are meant to be, I actually feel as though I'm there. So I can wholeheartedly recommend both of them in terms of accuracy. I would have preferred the E-G to be modelled pre-1982 so we would have had Eastfield loco sepot, Cowlairs carriage depot and Cadder marshalling yard too before they all closed but what's there cannot be faulted.
The class 67 addon is very good, but I would wait until it's fixed - it doesn't seem to like TS2012!
The class 60 is a very well modelled modern freight loco.
I absolutely love the UK steam addons and in particular the Jubilee, V2 and Tornado packs. But probably my favourite will be the GWR King class, to be launched next week complete with beautiful matching coaches. The first one built, No. 6000 (King George V) spent a month in 1927 on the Baltimore and Ohio Railroad and carries a commemorative US-style bell because of this.
This video demonstrates the beauty of the King class far better than I ever could:
http://www.youtube.com/watch?v=71ZhIRo9 ... ure=fvwrelGermany
The addons from virtualRailroads and Eisenbanwerk are of a consistently high quality and I can recommend any one of them!
I believe the Koeblitzer mountain route addon is good but I have no personal experience of it.
Hope this helps
John