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.