Roll a dice.. it's an individual thing really. I'd go for some ISO 400 film for starters, it's probably the most versatile.. so that narrows it down some. Then you can choose between HP5, Tri-x, Tmax, Neopan, and various others.. you could even go for a C-41 film like XP2 or BW400CN (which I found quite alright). I personally like neopan, but the truth is that there's no "bad" film, each of them just has different characteristics. If you're just after a roll or two I'd go to the local and just choose whatever catches your fancy.