Personally, I'd do Forest, if you're finding it too hard on Ruins, then you're probably spending more time healing than EXP'ing. Get better trained, then commence harder levels.
It depends if you wanna go rare hunting too, Obviously there are better items to collect in the ruins. If it's purely exp gathering I have to agree and say do Soul of a Blacksmith runs, that gives you more initial exp than Heat Sword.
And I just joined today and wanna say a hello to everyone

Well, welcome to the forums Azbats!
Endless Nightmare can get you a lot...
Hi azbats, thanks for the advice! It looks like i've drained the forest of rares, since when doing runs I only seem to find the same ones over. Apart from ones I ahven't got from hildetorr's and the dragon. I'll try doing that quest on ultimate then.
ttf lets you do all the bosses quickly if you want to get to the bosses...
enaD and ttocS u should do the forest because u can kill the boss and get a good bit of exp.
The highest amount of exp you gan get in the shortest time would be to do TTF.