Theoritically it should be Flash Initiator since its a role thats always with the pack, doesnt enter site himself (reduced pressure to frag) and can afford to focus more on utility even mid round, Scan Initiator also has the advantage of not entering site itself but nowadays its reserved to strong Closers (Trent, Riens) since your utility is vital both pre and post plant, way more than flashes.
Controller used to be the go-to IGL role since even with bad fragging your util alwqays has impact but nowadays you see more and more cracked Controller players (Keiko, Mako, Bang and many others) cause the meta ervolved in a way where the conmtroller is usually the second man into site after the duelist (often utilizing Omen TP) and having a strong aimer trading the duelist is very important.
Sentinel is another good IGL role cause the utility helps you apologize for bad mechanics and lets you get easier kills, also allows the IGL to get his own information and its probably the role that impacts the round the least if he dies untraded.
In the end it depends mostly on your teammates and you as a player but if I had to make a choice id say Flash initiator is by far the most fitting role for an IGL right now