Reanimator decks are looking to get a powerful creature into the graveyard quickly through discard or Entomb effects, then cast a spell to return it to the battlefield at a steep mana discount
The matchups become whether you know the opponent's deck, whether you have the answers for their combos, and whether you interact with their chains at the correct points

Aberrant Aether Channeler Agent of Treachery Alquist Proft, Master Sleuth Architects of Will Armorcraft Judge Azor, the Lawbringer Baleful Strix Barbed Servitor Barrin, Tolarian Archmage Battlewing Mystic Bazaar Trademage Be'lakor, the Dark Master Beastbond Outcaster Bedlam Reveler Biomechan Engineer Blade Juggler Bladecoil Serpent Blood Servitor Bloodcrazed Socialite Bloodtithe Harvester Bonny Pall, Clearcutter Bookwurm Borborygmos and Fblthp Briarbridge Tracker Broodlord Caldaia Strongarm Callous Bloodmage Carnage Interpreter Carven Caryatid Cavalier of Flame Cavalier of Gales Cephalid Sage Champion of Dusk Champion of Wits Charming Scoundrel Chronicler of Heroes Circuit Mender Citanul Woodreaders Clackbridge Troll Clattering Augur Clive, Ifrit's Dominant Clockwork Servant Cloud of Faeries Cloudblazer Cloudkin Seer Combat Thresher Combustible Gearhulk Common Iguana Conciliator's Duelist Coralhelm Chronicler Crackling Drake Crypt Lurker Cultivator Colossus Didact Echo Discerning Peddler Disciple of Bolas Disciple of Freyalise Discourtesy Clerk Doom Weaver Dormant Sliver Drownyard Explorers Dusk Legion Zealot Duskana, the Rage Mother Eagle of Deliverance Earthshaker Dreadmaw Echo Inspector Echo of Death's Wail Elite Guardmage Elite Instructor Elvish Visionary Errand-Rider of Gondor

It is great in any matchup except aggro, and even then it can help you stabilize if you can live long enough to stick a big demon