At the request of another member, I'm going to give my tips and tactics for proper threat assessment. Other people can add in their own advice, reinforce what has already been said, or ask questions.
Deck Building
First step is the deck building. Make sure you have a good interaction package, otherwise the rest of this thread doesn't matter.
Creature/Artifact Removal- Have 3-5 pieces of creature removal, and 3-5 artifact removal.
- Should be 3 mana or less. 4 mana isn't worth it (with very few exceptions).
- Instant-speed. This part is very important.
- Hits multiple supertypes. Artifact removal should hit enchantments when possible. Cards like Abrade (https://cards.deckstats.net/magiccard.php?utf8=1&lng=en&card=Abrade) and Hero's Downfall (https://cards.deckstats.net/magiccard.php?utf8=1&lng=en&card=Hero%27s+Downfall) are also good.
Counterspells- 3-5 counterspells. Because counterspells are inherit card disadvantage in multiplayer, you don't want to pack your deck full of them.
- 2 mana or less. This is often all you will have left after you pass your turn.
- Countering non-creature spells is more important than creature spells
- Simple package: Mental Misstep (https://cards.deckstats.net/magiccard.php?utf8=1&lng=en&card=Mental+Misstep), Arcane Denial (https://cards.deckstats.net/magiccard.php?utf8=1&lng=en&card=Arcane+Denial), Swan Song (https://cards.deckstats.net/magiccard.php?utf8=1&lng=en&card=Swan+Song), Counterspell (https://cards.deckstats.net/magiccard.php?utf8=1&lng=en&card=Counterspell), Mana Leak (https://cards.deckstats.net/magiccard.php?utf8=1&lng=en&card=Mana+Leak).
Stax effects- Make sure they are relevant in your meta. Don't use Cursed Totem (https://cards.deckstats.net/magiccard.php?utf8=1&lng=en&card=Cursed+Totem) if it's not going to help. Be sure to do a meta-analysis (look at the strategies of the other decks you'll face) before deciding what stax effects to use.
- Be sure to have some way to mitigate the harm that stax will do to you. If you're using Winter Orb (https://cards.deckstats.net/magiccard.php?utf8=1&lng=en&card=Winter+Orb), make sure you have creatures (Elvish Mystic (https://cards.deckstats.net/magiccard.php?utf8=1&lng=en&card=Elvish+Mystic)) and/or artifacts (Talisman of Indulgence (https://cards.deckstats.net/magiccard.php?utf8=1&lng=en&card=Talisman+of+Indulgence)) that give mana.
- Not all decks need stax effects, but grave-hate is usually a good idea.
General Rules
- Be cautious with politics. People will betray you if it means winning the game. Always do what is best for yourself.
- It is usually a good idea to wait as long as possible before using removal. As annoying as Notion Thief (https://cards.deckstats.net/magiccard.php?utf8=1&lng=en&card=Notion+Thief) is, someone else playing Kiki-Jiki, Mirror Breaker (https://cards.deckstats.net/magiccard.php?utf8=1&lng=en&card=Kiki-Jiki%2C+Mirror+Breaker) and then winning with a combo is worse.
- Prioritize targeting the person in the lead. If they have a big advantage over the other players, gang up (https://cards.deckstats.net/magiccard.php?utf8=1&lng=en&card=Gang+Up) on them. Your best bet is to remove their card advantage (draw outlets like Rhystic Study (https://cards.deckstats.net/magiccard.php?utf8=1&lng=en&card=Rhystic+Study), or their commander if that provides them an advantage).
Dealing with combos
As you face off against more combo decks, you'll start to see signs as to when they are about to combo off. If they played Bloodchief Ascension (https://cards.deckstats.net/magiccard.php?utf8=1&lng=en&card=Bloodchief+Ascension), there is a good chance that they will play Mindcrank (https://cards.deckstats.net/magiccard.php?utf8=1&lng=en&card=Mindcrank) in a turn or two, so get ready to use that Nature's Claim (https://cards.deckstats.net/magiccard.php?utf8=1&lng=en&card=Nature%27s+Claim). Don't use it right away though, wait for them to actually cast Mindcrank (https://cards.deckstats.net/magiccard.php?utf8=1&lng=en&card=Mindcrank). That moment is the best time to remove Bloodchief Ascension (https://cards.deckstats.net/magiccard.php?utf8=1&lng=en&card=Bloodchief+Ascension), because someone else could play their combo leading up to that point.
With combos, your top priority is stopping the fastest decks first. This might sound obvious, but you should always be wary for the person who is going to combo first. Think short-term. The other important thing is using stax effects. Stax cards are really good at stopping combos. If you face a lot of artifact-based decks, consider using Stony Silence (https://cards.deckstats.net/magiccard.php?utf8=1&lng=en&card=Stony+Silence) or Aura of Silence (https://cards.deckstats.net/magiccard.php?utf8=1&lng=en&card=Aura+of+Silence). If they win with a creature's activated ability (Like Kiki-Jiki, Mirror Breaker (https://cards.deckstats.net/magiccard.php?utf8=1&lng=en&card=Kiki-Jiki%2C+Mirror+Breaker) or Prime Speaker Vannifar (https://cards.deckstats.net/magiccard.php?utf8=1&lng=en&card=Prime+Speaker+Vannifar)) use something like Cursed Totem (https://cards.deckstats.net/magiccard.php?utf8=1&lng=en&card=Cursed+Totem) or Linvala, Keeper of Silence (https://cards.deckstats.net/magiccard.php?utf8=1&lng=en&card=Linvala%2C+Keeper+of+Silence). If they cast lots of spells in one turn, use something like Damping Sphere (https://cards.deckstats.net/magiccard.php?utf8=1&lng=en&card=Damping+Sphere), Rule of Law (https://cards.deckstats.net/magiccard.php?utf8=1&lng=en&card=Rule+of+Law), or Ruric Thar, the Unbowed (https://cards.deckstats.net/magiccard.php?utf8=1&lng=en&card=Ruric+Thar%2C+the+Unbowed).
Dealing with stax
Most important thing with stax effects; if it's stopping someone else from winning, leave it alone. As annoying as Damping Sphere (https://cards.deckstats.net/magiccard.php?utf8=1&lng=en&card=Damping+Sphere) can be, if it's stopping a combo deck from winning, leave it be. You should only remove a stax piece if it is either hurting you more than other players, or if it is directly stopping you from winning. As with combo pieces, wait for the last possible moment to remove a stax piece. This will usually be right before your untap step (end step of the previous turn) so that way you are the first person to take advantage of the stax piece being gone.
Baiting and bluffing
This applies more to counterspells than removal, but it can be applied to both. Baiting is when you play a spell, expecting it to get countered, and then you play the spell you really wanted to cast. For example, if you're playing Brago, King Eternal (https://cards.deckstats.net/magiccard.php?utf8=1&lng=en&card=Brago%2C+King+Eternal), you might cast Armageddon (https://cards.deckstats.net/magiccard.php?utf8=1&lng=en&card=Armageddon) (your bait), which gets countered, and then you can cast Panharmonicon (https://cards.deckstats.net/magiccard.php?utf8=1&lng=en&card=Panharmonicon). Depending on the situation, you might instead use Panharmonicon (https://cards.deckstats.net/magiccard.php?utf8=1&lng=en&card=Panharmonicon) as the bait, and then cast Armageddon (https://cards.deckstats.net/magiccard.php?utf8=1&lng=en&card=Armageddon). The point is, your opponent uses their counterspell (https://cards.deckstats.net/magiccard.php?utf8=1&lng=en&card=Counterspell), so you can safely cast your other spell. Keep in mind that your opponents might not have a counterspell (https://cards.deckstats.net/magiccard.php?utf8=1&lng=en&card=Counterspell), so make sure that you're not hurt if your bait actually resolves.
Bluffing is when you pretend to have a counterspell (https://cards.deckstats.net/magiccard.php?utf8=1&lng=en&card=Counterspell), but you don't actually. You make your opponents think you have one, so they hold back their most powerful moves. Games can be altered by bluffing. One of the best bluffs:
(https://i.imgur.com/A9b96VK.jpg)
I'm not kidding, this is often all it takes to make people second guess (https://cards.deckstats.net/magiccard.php?utf8=1&lng=en&card=Second+Guess) their plays. Now if someone plays something that is definitely worth countering, play it off like you don't need to. "I'm casting Purphoros (https://cards.deckstats.net/magiccard.php?utf8=1&lng=en&card=Purphoros%2C%20God%20of%20the%20Forge&set=THS)." "How many cards do you have in hand?" "Three." "Yeah that's fine." Make it look like you have a plan to deal with this obvious threat, so it isn't counterspell (https://cards.deckstats.net/magiccard.php?utf8=1&lng=en&card=Counterspell) worthy. They might be baiting, and this will still make people hesitate.
Wrapping up
I'm done for now. Bring on your questions and comments. In a little while, I might post little challenges to test your threat assessment skills.