[[url=http://[/URL]="http://www.knufinke…"]SIR[/]="http://www.knufinke…"]SIR[/] is very good, and very free.

Best and Cheap should never be used in the same sentance because you can never have both in the same product. The best of anything in audio is never cheap, and anything that is cheap, is never best.

Your better off asking, who currently makes the best reverbs today or telling us what your budget is and then asking what reverbs are available that you can buy that are within your budget.

Take the SIR or similar and fill it with http://www.samplicity.com/. Affordable but awesome impulses. You'll not regret it 99%. Peter Roos made it very good.

ITB or external?

For an external device, you might try to snag a Nanoverb on eBay... but as AudioGaff said; The good suff ain't cheap, and cheap stuff ain't always good.

Gotta budget?

Max

Also good and cheap external is the LExicon LXP-1.

You can find one for about $75 and it sounds much better than their new cheap stuff.