It depends which era and style of blues and soul you're looking for. If you're looking for folk blues, Catfish Records have released a three disc "definitive" Lead Belly box. You may also want to check out Mississippi John Hurt and Robert Johnson, both of whom are easy enough to find in any decent Blues section.
Chess have a very good reissue programme for their artists, including John Lee Hooker, Muddy Waters, Buddy Guy and Howlin' Wolf. If you pick up the Chess remasters of those guys you're not going to go wrong. The Willie Dixon one is especially good. Plus the Chess 50th anniversary collection is flawless.
Soul albums are always tricky, especially those that feature artists from the 50's and 60's, as the artists and record companies put more effort into producing singles, and albums were merely a vehicle for the current singles with added filler. You should get (unless you already have them, that is) the 3cd Motown 60's comp, and best ofs from James Brown, Stevie Wonder, Aretha Franklin, Wilson Pickett, Otis Redding and Sam and Dave. James Brown Live At The Apollo I and II are pretty essential, and available in deluxe edition these days. Also Otis Blue by Otis Redding, and if you can get your hands on it, Signed, Sealed, Delivered by Stevie Wonder (which contains the definitive version of Baby Don't You Do It).
There are some good label comps out there as well. Invictus Chartbusters, featuring the later work of Holland Dozier Holland and their proteges, such as Freda Payne and the Chairmen Of The Board is really great. The Brunswick Story features major hits by Jackie Wilson, the Chi-Lites and many others, and the Curtom label comp charts the career of Curtis Mayfield and others.
There are plenty good Northern soul albums out there, but there are also loads that should be clearly labelled "hardcore collectors only". The best bet is try before you buy. Sad to say, the best one available right now is the Best Northern Soul Album In The World...Ever! Buying all the 7"s contained within would set you back several hundred thousand pounds, so it's not nearly as bad as it sounds.
Hope this helps.
