I like these monuments, and have played with a few of them at least a few times:
Oketra's Monument is amongst the strongest. White lacks clean ramp so this is a fine enough "3 mana rock" for a monowhite creature deck. A free dork on any creature cast, discounted or not, can be pretty big and gives this a home in both Aristocrat-y style decks and Token decks, which covers a significant amount of the territory white likes to play in. I'd call this borderline mandatory in a monowhite deck with any appreciable creature count, and very good in most two-color white decks with a CMC 4+ Commander. It would still stay playable in 3 color decks with a CMC 5+ Commander, so long as a decent percent of the creatures were white. I've played this in several spots and it's been really solid.
Bontu's Monument is also pretty solid, but at its best in a bleed-ery deck. There are plenty of aggressive B/x decks (like
Edgar Markov vampires or
Yuriko, the Tiger's Shadow ninjas, where taking a turn off to cast this just isn't worth it and won't pay for itself. For perspective, with the delay I'd say that Oketra's needs to trigger at least 4 times to be better than just jamming
Spectral Procession if all you cared about was bodies, which feels reasonably likely with a round or two around the table. This is "just damage" so it definitely needs to trigger closer to 5-6 times to pay for itself, and that just isn't for every black deck. Awesome in an aristocrat-y deck that can use it though and relevant enough to derpier black tribal lists (Rats, Demons). I think the price is due to it being a mandatory piece of
Acererak the Archlich which is only at 245 decks but is a fun little hipster build.
Hazoret's Monument seems a bit overrated to me. I run cards like
Thrill of Possibility and
Cathartic Reunion mostly as a matter of necessity in Red, Gruul, Boros, and some Rakdos decks as rummaging is always really awkward. This also never really "pays for itself" by itself - so you really need it to be in decks that care about discarding cards or care about filling their 'yard as otherwise you're spending 3 mana in, say, a goblin deck to just have some handfixing later. Don't get me wrong, this is still a solid card and is eminently playable in monored decks with 25+ creatures and a more midrange plan as well as discard-centric decks like Minotaur Tribal or Madness. But having tried it even in decks where it ought to be pretty great like
Grusilda, Monster Masher reanimator,
Anje Falkenrath Madness,
Wort, the Raidmother spellslinger, and
Daretti, Scrap Savant artifacts and legends, it has been a bit underwhelming.
Rhonas's Monument is where the line goes from "fairly playable" to "not super playable". Part of the problem is that green is THE ramp color, so a 3 mana pseudo-rock/creature-only
Emerald Medallion isn't as great for monogreen as the first three were to monodecks. Part of it is that the trigger isn't all that useful, giving a one-shot boost to a single creature. This is at its best in a deck that runs a lot of little dorks but is open to a voltron plan, or a trample tribal deck with
Stonebrow, Krosan Hero...but even there, I'd likely be looking to
Garruk's Uprising for my trample needs and
Sylvan Anthem as my "pay off for playing a ton of small dorks".
Kefnet's Monument is the loser of the cycle. It's not uncommon for a blue deck to run precious few creatures as the spell color. Blue decks with a heavy creature count are often going to be merfolk, rogue, ninja, or artifact decks where the discount is unlikely to matter. Even if the discount does matter (in a Faerie, Sphinx, or Seamonsters deck) the trigger itself is hyper conditional. I would still probably give it a look for really top-heavy creature decks in monoblue or Simic/Izzet, like Sphinx and Seamonster, with a special nod to seamonsters since they're likely to have some ways to force the tapping issue.