Fuji Acros or TMax for ISO 100 would be your best bet for sure.
They are indeed the "'best" concerning grain and resolution of all the 100 ISO films. :P
Don't use Rodinal if you want smaller grain is my second tip, my experience with Acros is that it is very low-grain in most normal developers I've tested it in so far (Tmax dev, Tetenal Ultrafin, HC-110).
If you can get hold of Efke/Adox 25 ISO, it will probably be the cleanest no-grain film possible (except microfilm) but I would think that grain rarely is a problem in 120 with Delta, FP4, Acros and TMax.