For me I'd order it
1.Blazing Sword
2.Sacred Stones
3.Sword of Seals
A lot of things bother me about sword of seals, I didn't like how the only chapter objective was seize, I like rout and defend missions. Roy does not promote until very late in the game. Idenn, is absurdly easy for a final boss, in fact she's probably the easiest final boss in any game ever. Roy is the only one who can marry through supports and he can marry everyone, this isn't too big a complaint, but I do find it slightly irksome. I also found the characters to have received little development compared to 7 and 8, which is probably what bothers me the most about the game. 6 did have my favorite recruitment theme though.
As far as 8 goes, 8 was the first game I played, but the traversable map bothered me, being able to infinitely level made the game too easy. I thought the game was on the short side as well. Creature campaign and the Lagdou ruins was pretty cool though.Aall in all I just liked 7s music, characters, story and style better. The only thing that bothered me about 7 was the lack of a manakete class, but that's not really the end of the world. 7 was longer and harder than 8.
So in summary,
6 was too rough, lacking some features and nuances, 8 was too short and easy and I feel has an inferior plot and soundtrack to 7
7 is the winner in my book