My favourite retailer is East Coast as I like the "Mixing Deck" interface - I find this very easy to use, although it could confuse others.
Although Southern and Southeastern have the same interface, I only use their website to buy discounted tickets on their own services. Reason being they get difficult if tickets get lost and you didn't pay for recorded delivery.
Recently I've been buying through
FTPE» and
SWT▸ (both Trainline websites) as they are offering cashback on their tickets through cashback websites (like Top Cashback, imutual, Fatcheese, etc). Martin Lewis has a good guide on how these types of websites work:
http://www.moneysavingexpert.com/shopping/cashback-websites