Yep 8) havent played dual destinies sadly...
Nah don't start with 7, it may be the best selling, and heck, it is an awesome and good game, but it is not the best. Try 6, not only it is easier (GBA or SNES) but it is the best :] Tbh, it all depends on you. 6 is personally my fav, but what do you like most in video games? Gameplay,story..?
I see you like Phoenix Wright, which must mean you like story soundtrack and characters, if you like that then 6 is definitely what fits you most :) If you like sci-fi a bit more than Fantasy, then try 7 or 8.