Well, most of the Railworks add-ons available are excellent and you would'nt be disappointed with any of them.
Alot of add-ons are available on Steam some are not, here is a list of sites I visit regularly, they produce payware expansions for Railworks, some of their work is available on steam some of their work is only available directly from their web site. Generally the addons are slightly cheaper direct from the authors web site or may contain bonus extra's:
For UK rolling stock, routes, scenario packs etc some of the main sites to visit:
http://www.JustTrains.net
http://www.oovee.co.uk
http://www.ironhorsehouse.co.uk
http://www.armstrongpowerhouse.co.uk
http://www.digitaltraction.net
http://www.meshtools.co.uk
I strongly recommend that you look into the Armstrong Powerhouse sound packs for your favourite loco's, especially the HST (Valenta) Sound pack, it is indespensable. There are You-Tube clips on the web site showcasing the sound upgrades.
All the Railworks addons available at the above sites are worth any penny but some of my personal favourites are:
Class 20 Pack from
http://www.justtrains.net, if I was only allowed to take one Railworks addon to a desert island this would be the one I take. £20 it is expensive but the locomotives are beautifully modelled, the sound are first class and you can drive these Class 20's from both cab positions. It also comes with lots of liveries and 9 scenario's and includes 2 wagons: the Nuclear flask wagons and the Rail Head treatment wagons. Check the web site for YouTube clips. A very comprehensive pack indeed.
I can confirm as I also have it that the IronHourseHouse Class 20 with the Armstrong Power House Class 20 Enhancement pack is just as good, works out cheaper but you get less scenario's, less liveries and you cant drive this model of Class 20 "nose first". With the Class 20 enhancement pack installed, quite possibly the best bogie flange squeal I've heard yet. Even though the sounds are slightly better I think the Just Trains pack offers more value for money overall.
Personally though if you can afford it buy both class 20 packs.
Also I highly recommend the Digital Traction Mk2e coaches (and the Mk 1 coaches as well)
Oovee Class 57 (direct from Oovee or from Steam) Lovely Loco to drive
or
another realy nice modern (well reasonably modern) loco is the Class 60 from Just Trains.
The iconic class 56 from IronHorseHouse is great too. Al the IHH content is great, most of it is available from steam but you get extra's if you buy direct from the IHH site.
Another of my favourite loco's is the MeshTools Class 50 with the Armstrong Powerhouse Class 50 sound pack. I dont have the IHH version so cant comment on it.
Finally if you like DMU's, the CLass 101 Pack on Steam is excellent.
Cant comment on electrics or steam as I am a BR diesel man thru and thru.
Hope that helps and remember its just my opinion what I think is best.