basically, what percent of encounters should be really epic and crazy? What encounters should be skirmishes with kobolds. And how do I run these epic scenes?

Use smaller encounters to drain party resources so they're not going into the epic and crazy encounters at 100% and breeze through it.

Toy with the players. Make them think one thing about an encounter, then reveal another, or let something seem significant only to have it come to nothing. You know, like real life, but with fantasy elements.
Rule of thumb: As long as encounters are interesting, make sense within the story, and fit the strategy and tactics of the creatures in question (read this blog for tips on combat styles: http://themonstersknow.com/), then you're doing fine. I play it by ear, and if encounters are coming off a bit more weak than I'd like, there's always a fresh wave of reinforcements or a shocking reveal. And if it's going south fast, there's always a prison break, or a last minute rescue from an unexpected ally. :)

A lot of this comes down to personal preference.  I don't care too much for encounters that exist for no purpose other than being annoying speedbumps.  I use very few of those, preferring big 'set piece' encounters.

I think of and plan out campaigns like a flowchart. Since no DM can ever predict what the players will do or where their characters will go, you as the DM must be prepared for anything.

But that doesn't mean fleshing out every square inch of the world and every person in it. Instead, you just need to plan what the BBEGs will be, and roughly where you want them to show up, both geographically and in terms of party level. Those boss fights and BBEG encounters are the crazy/epic battles. Now that you have a handful of these planned out, you simply need to connect the dots. There could be several different paths that give the players the illusion that they are free to make any decision and go anywhere at any time, but really - behind the scenes - you know that all of those paths will inevitably converge on the BBEG encounter that you already planned. 

So along those divergent paths there will be different kinds of minor encounters:

1. There will be random encounters along roads and in underground caverns and such. These shouldn't really challenge the party too much, but they keep the party from becoming complacent. Five goblins ambushing a 5th level party in the woods is no challenge. But if you play it right, it can really unnerve the players. They're getting hit - they can't see the attackers - they don't know how many there are or what they are - some want to fight some want to run - send in a wolf or a mastiff and I guarantee you'll see panic on their faces. Random encounters are intended to remind players of the need for perception checks, which slows them down to a normal travel pace, which can give the BBEG more time to carry out his evil plan.

2. There will be encounters instigated by the players. These are not planned. Maybe they pick a fight in a bar. You didn't plan that, they just did it! Well, that costs them resources and time. Maybe the rogue gets caught shoplifting and is held overnight until the magistrate arrives. Now the party has lost a day or two of travel time, and they're out probably a few dozen or a few hundred gold pieces. Side quests also fall into this category. If a player wants to buy an item but can't afford it, so the shopkeeper says "go do this job for me and I'll give you a discount", that's a side quest. It's not directly related to the campaign's plot, but it costs the party time and resources. Those should be a bit of a challenge, but should also contain some clue for the players to indicate that they need to get back on track.

3. There will be encounters with minions of the BBEG! This will probably be the majority of fights the party has. The BBEG doesn't fight his own fights. He has minions for that. And the BBEG probably has some way of knowing that the party is coming after him, either through informants on the ground or scryng spells, etc. So he knows what to put where. It's a chess game. He moves his pieces, the party moves, he moves another piece, etc. These encounters should be challenging, because the BBEG honestly wants to stop the characters. The bad guy knows that the party will eventually find him, he's just trying to stack the deck in his favor. He wants the party to fight him on HIS turf, on HIS terms. He wants to ensure that the party has already depleted many of their spell slots and such before that fight begins. And he will attack by surprise if he can. And it won't just be him; he'll have minions with him, and traps, and terrain advantage, and knowledge of the secret passages in his lair. With all of those advantages in his favor, the BBEG doesn't need to be Tiamat or Orcus. Even a mediocre bad guy can use the force multipliers of "minions, maps, and movement" to seem much more powerful than he is. 

Finally, you should also plan a huge battle or two where the players are not the center of attention. Maybe the BBEG sends an entire army against a city. Think of the Battle of Helm's Deep, or the Siege of Neverwinter. Sure the party is in the fight, but they're not the center of the fight. They need to feel surrounded. They need to feel overwhelmed. This serves two purposes: First - it will make them plan ahead better, because they won't want to feel that way again, and Second - it will remind them that they are not alone. They are not just five random people doing their own thing. There's a whole world out there. People are dying! People are losing their homes! The players need to feel responsible! This will help keep them focused and on track. 

Well, that's pretty much the "what" and the "where". My only other advice would be to remember to think in three dimensions. The difficulty of an encounter is more than just a comparison of the party level versus the creature's CR. Remember that melee players aren't much good against flying monsters. Remember that players on a ship at sea need to do more than just defeat the wyverns, they also need to protect the sails!!! Imagine winning the fight and then finding out the sails are shredded and you're stranded at sea! 

Kobolds are usually considered low-level trash. BUT! Imagine this - you're in an icy cavern, the floor is slippery, movement is half speed or else you need a DEX check to avoid falling. There's a cavern and the only way across is a narrow icy bridge 60 feet long. Single file, half speed, falling means death, the party slowly carefully gets halfway across and then BOOM a dozen kobolds pop out of a tunnel and start firing arrows at the party! Now what??? Monsters will use whatever terrain or situational advantages they can get, just like players do. Remember to utilize them! Think in all dimensions. When your players enter a room or a cavern, one of them should just instinctively say, "Check the ceiling!" or else you're not challenging them enough. 

Keep them guessing. Keep them moving. Keep them on edge. 

I hope this helps. :-)

I second Tayn's advice, but let me add that skirmishes with kobolds can feel epic if a whole wing of red dragons are firebombing a city in the background...

Think about the Battle of Takodana in Force Awakens.  On paper, what happens is a party of 4 characters kill about 8 trash mobs and then one character gets captured by the BBEG.  

On screen, everyone looks up in the midst of personal character drama, sees half a dozen planets exploding, then TIE fighters strafe a thousand-year old temple, all of the characters have a badass hero moment, things look hopeless, then the X-Wings show up and save the day, Rey gets kidnapped and the stakes go up 1000%.  And that's a minor fight at the end of Act II.  Epic and crazy isn't necessarily a function of the dice.  It can also be a function of the writing.

Also, google "Tucker's Kobolds" some time.  When it comes to a challenging fight, there are no trash mobs, just trash tacticians.