Nocturne's the best start for the mainline unless you just have a major hard-on for Wizardry, in which case either SMT or SMT2 would be fine.
I'm in the minority here on /v/, but Persona 1's gameplay is straight ass. That battle grid is awful. P3/4 play more like traditional JRPGs with SMT elements, which makes them more widely palatable, but both games have ungodly slow, uneven story pacing.
As I have to do in all these "new to SMT" threads, look into Digital Devil Saga for straight JRPG gameplay in an SMT-themed universe, Devil Survivor for tactics gameplay in an SMT-themed universe, and the Devil Summoner Raidou games for action combat in an SMT-themed universe. If you like any of the mainline games you should also look into Soul Hackers and Strange Journey, both of which are stellar SMT games but a bit too hardcore for newcomers.