I think it really depends on what you mean by "fun", and how willing to stretch the 18yo male protagonist.
If fun just means on your toes excitement and generally being a good read, I can definitely second Wheel of Time. It's an older book, but reads like modern fantasy. Lightbringer is also a good one, though I haven't read the entire series.
If fun means more lighthearted or a positive outlook, that's not quite my expertise, but I have heard many good things about Discworld from many different people.
If you want to loosen the 18yo protagonist, lightning round! (There's a lot of Sanderson, but I do love his stories)
Mistborn, by Sanderson: 18yo(ish) female protagonist, not your typical YA young girl though
Stormlight Archive, by Sanderson: Multiple POVs, two maim ones being 18yo male and female. Great story, as others recommend.
Kings of the Wyld, by Nicholas Eames: Follows an older protagonist, more of a mature tone, but it kept me hooked until the end.
I also have some scifi recommendations if you want those.