...and yet another vote for Stern Lightning.
First; it's pretty hard to actually get multiball. Second; when you finally do, re-locked balls stay in their lock instead of getting kicked back out. Third; this machine is so fast 'n furious, it's hard enough keeping 1 ball on the playfield. Let alone 2 or 3.
However, multiball rules stay active during the rest of that ball, even in one-ball play.
That said, Lightning is an extremely fun package. I'm hoping someday somebody re-programs the multiball rule so that re-locked balls get kicked out again and the multiball rule ends when only one ball is left on the playfield.