I love Aberjack but I would not use him in his current situation.
If you’re okay with frozen, you can get Alligator Fontaine, Mighty Magic, Grafenstoltz, and Jaguar Mail.
If you’re looking for fresh, an event “specialist” is harder to come by because (as about 500 threads have pointed out) US breeders just don’t breed enough event horses to incentivize stallion owners in the US.
Riverman is available and has produced upper level horses if you’re open to Holsteiners.
Likewise, Concerto Grosso has some promising up and coming youngsters but he’s a BIG boy and I personally would want a refined mare. I have two foals coming from him out of a BWP registered Holsteiner/TB cross, but she’s about as heavy as I’d want to go with him. He’s had some lovely crosses on TB mares.
Cascani produced Fleeceworks Cinco, who’s had a great run in the YEH classes, although he has to breed on site or same day counter-to-counter semen shipments.
Out of the box, I like Lotus T. He’s over 50% TB (Approved AHHA & RPSI). His babies have typically gone to hunter/jumper careers, but I’ve seen quite a few and they can gallop and jump and seem to have good minds.