Don't have much for it, I only have the main book, the campaign books & the Finian Clan-book.
I would say my favourite has to be the campaign books at the mo, if singling down to a single book, probably the first, but that's because I emphasise low-level play.
Out of left field with a spin ball, but I'd say the best suppliment for Slaine is Tir Na Nog and then the Horned God campaign.
Tir Na Nog give enough information to actually get a game going, and to incorporate some of the more nasties of Slaines World - the Midguard, Skull Swords, Feg and several supernatural nasties are included, along with enough details on the tribes to include them (the tribe books are nice, but not really all that necessary).
Still it needs a guide to the villianous creatures (Els, Formorians etc...)
I really think the Fir Domain is the best supplement for Slaine so far - its well written, with a solid and consistent theme, great (new) illustrations, and well thought-out character concepts. I highly recommend it - its also a great adjunct if you are playing the Moon Sow campaign.
Best supplement for the Slaine-rpg?
Definitely the Tir Na Nog-book for the obvious reasons that it gives a lot of useful info on clans,opponents and even relations between the tribes.
If i have to be totally subjective and allow my feelings for the different tribes get in the way of a solid opinion based on objective matters i'd have to say the sessair- and falian-sourcebooks.