Westing is a fun antagonist because, even though we never meet him as himself (just see him disguised as other characters), he's the reason the book's whole plot is set in motion. Finding an heir to his massive estate is his biggest goal, and he's not going about it the easy way. He fakes his own death (twice!), pits several unsuspecting innocents against each other, and launches an intense competition to see who will win his fortune. Technically, he causes disappointment for many of the characters, since they think no one ever won the game and the $200 million slipped away from them.
For many characters, Westing is also a personal antagonist: the Judge resents him for what she thinks is trying to buy her legal loyalty, while Mr. Hoo's never stopped hating him for that little incident with the disposable paper diapers. But he didn't really hurt anyone, and he's by no means the worst bad guy you'll ever see. In Westing's defense, we see him working pretty hard – in disguise, of course – to fix his past wrongs.
SPOILER ALERT! In some ways Sandy's even more of an antagonist than Westing in terms of leading people astray or pointing them in wrong directions, even though they're the same person and therefore could be considered the same antagonist. Yet Sandy's deceit can be felt more personally than Westing's because nearly all the characters know him as Sandy first, Westing second. He even fools his ex-wife into thinking he's someone else.
Sandy's the game's ringer – disguised as yet another disgruntled player competing for Westing's fortune – but he's continually positioned in the perfect place to throw everybody in the wrong direction. He interrupts the reading of the will at a key moment, he leads the Judge in the direction of Otis Amber, and he keeps her from investigating the one player she should really have looked into. Plus, he also fakes his death, which is a total antagonist giveaway.