Pre-2005 there was such a wealth of 100-125 speed films available (I used them all) but had particular fondness for Agfa APX100, Neopan SS, FP4, and Verichrome Pan. When those 4 were available I hardly ever bought Plus-X and never touched Tmax 100 or Delta. Among them I'd say Neopan SS was the closest to Plus-X in terms of grain & contrast, though it was really a somewhat faster film than the 100 box speed.
There really isn't anything out there now that's a real close match Plus-X (or any of the other defunct stocks). But there are some good ones: Kentmere 100 as someone else pointed out. In fact it is my favorite all-around 100 speed film right now. It's actually a little better than Plus-X I feel. Too bad it's not made in 120.
I gave Delta 100 another look and was pleasantly surprised recently. It may have to do with me doing more scanning and less printing today than I did 20 years ago. I had trouble printing both Delta, Tmax 100 and Plus-X all due to blocked-up highlights. I never went to the trouble of trying to tame them with different asa ratings or developers since all those other aforementioned films printed so easily with great highlight detail.