7Aliens Catanya is very powerful. Windows-only. On Mac I use Numerology, it's technically a modular step sequencer environment but it makes a nice programmable arp The Proteus 2000 line has an awesome arpeggiator, too, and can sometimes be had for about the price of a plugin lol.
"the best" is pretty subjective, though. Download whatever demos you can and see which you like most.
The built in arpeggiator that comes with the access virus is designed to go out of synch for that " i love it when my entire track is ruined by an eight beat loop " feel that I so crave . All my problems are solved with a separate midi insert arp so I will try Nora