Risk is the most-played and most-criticised conquest game in the world. The base strategy is real — secure Australia, build continent bonuses, manage your reinforcement rate — but the execution suffers from extreme length and brutal player elimination.
The dice variance is famous: a 30-army stack can lose to a 10-army stack on a bad roll streak. The community-recommended "blitz" rule (auto-resolve all dice) shortens the game noticeably.
Risk: Legacy (2011) is a permanent rewrite that fixes most of the base game's problems and is one of the highest-rated games on BoardGameGeek. Skip the original; buy Legacy.