If you like the stagnation of a two-party system, and want third parties to always have a near-zero chance of winning, then yes, plurality voting is the best system. Those two parties can consolidate their power and permanently keep out third parties, regardless of how corrupt/disliked the two parties become.

Instant runoff, among other things, keeps major parties on their toes and more responsive to the electorate's needs, since if voters are sufficiently angry they have a realistic option of putting a minority/third party in power.