I always thought 3. f3 d5 was the most forcing since opposite-side castling makes every move critical, but you need to have the Samisch KID in your repertoire.

don't think the various Exchange lines are the most forcing. Black always has solid playable sidelines where he's only slightly worse. 

Perhaps Bf4 lines can also be very forcing, e.g. 4. Bf4 Bg7 5. e3 c5 6. dc5 Qa5 7. Rc1 dc4 8. Bxc4 0-0 9. Ne2 Qxc5 10. Qb3 Nc6 11. Nb5 Qh5 12. Ng3 Qh4 13. Nc7 is one line i looked at for a very long time before concluding Black was fine a few years ago, but maybe Rybka can tell us new things about the position! also i remember looking at 4. Nf3 Bg7 5. Bf4 0-0 6. Rc1 c5 7. dxc5 dxc4 8. e4 Qa5 9. e5 etc. 
Bf4 generally forces Black to know highly-specific sequences without getting a dreadful game. Another forcing line is 5. Bg5 Ne4 6. cd5 Nxg5 7. Nxg5 e6 8. Qd2 exd5 9. Qe3+, but again Black can hold if he knows his stuff.

Finally, for something less explored, but also quite forcing, try the various Qa4+ lines e.g.  3. Nc3 d5 4. Qa4+ , or  4. Nf3 Bg7 5. Qa4+ or  4. cxd5 Nxd5 5. e4 Nxc3 6. bxc3 Bg7 7. Qa4+, imho Black needs to know some exact moves without being worse and even then there's lots to experiment.

With regards your final question, I'm fairly sure that objectively best is 7. Bc4. Sakaev knows best