Almost certainly Hufflepuff.
Hufflepuffs are hard-working, friendly, loyal, honest, impartial.
During school, Seasmus was rather easy-going and good natured. He was also a bit clumsy and had a pentachnt for setting things on fire (or outright blowing them up in the films). He displays curiosity from time to time (such as asking Nick how he could be "Nearly Headless"), but never enough to classify as a closet Ravenclaw.
In the fifth year, he was the least enthusiastic supporter of Harry and Dumbledore in his dorm room (Harry, Ron, Dean, Neville, Seamus). Though once he realized he was wrong, he apologized to Harry.
And finally, in the seventh year, he chose to stay at Hogwarts and oppose the Death Eaters.
So while Seamus demonstrated the "daring, nerve, and chivalry" that, according to The Sorting Hat, "set Gryffindors apart". He also showed that he "might belong in Hufflepuff, where they are just and loyal,".
‘Oh, you may not think I’m pretty,
But don’t judge on what you see,
I’ll eat myself if you can find
A smarter hat than me.
You can keep your bowlers black,
Your top hats sleek and tall,
For I’m the Hogwarts Sorting Hat
And I can cap them all.
There’s nothing hidden in your head
The Sorting Hat can’t see,
So try me on and I will tell you
Where you ought to be.
You might belong in Gryffindor,
Where dwell the brave at heart,
Their daring, nerve and chivalry
Set Gryffindors apart;
You might belong in Hufflepuff,
Where they are just and loyal,
Those patient Hufflepuffs are true
And unafraid of toil;
Or yet in wise old Ravenclaw,
If you’ve a ready mind,
Where those of wit and learning,
Will always find their kind;
Or perhaps in Slytherin
You’ll make your real friends,
Those cunning folk use any means
To achieve their ends.
So put me on! Don’t be afraid!
And don’t get in a flap!
You’re in safe hands (though I have none)
For I’m a Thinking Cap!’
Philosopher's Stone - Chapter 7: The Sorting Hat