Ike

Standard defensive build.
Weapon: Ragnell is the best sword for Ike hands down.
Support: Reposition is used to move units out of danger and keeping Ike in the front line.
Special: Bonfire allows to make use of that massive defense (can also use Draconic Aura since there is not a difference)

For skills:
A: Fury 3 is optimal, for survival since there's not a lot of Spd to work with (unless you have a +Spd). Also allows for better survival against mages that can be countered thanks to Ragnell.
B: Quick Riposte 3/2 is used to kill on defense, making the Spd stat not that relevant (you shouldn't be doubled a lot)
C: Hone Atk 3: since Ike hones a very high attack, he should be paired with someone with Hone Spd 3 to avoid doubles. Hone Atk 3 is used to have synergy between these two units (expecting Support from one another).
S: Quickened Pulse is used to always kill on defense with Quick Riposte and Bonfire no matter if you're hit once or twice.

Support: I personally pair Ike with Azura, but also buff him with Delthea (both girls with Hone Spd 3, Delthea with Dark Aura). This allows for easier baits, and can even kill some blue units taking little damage thanks to the huge defense.

This build puts Ike's natural bulkiness and defensive abilities to good use. With the setup of Steady Breath and Ragnell Ike can bomb any character that fails to one shot him with a well placed Ignis similar to his Brave variant though without having to sacrifice distant counter. Ignis can be swapped with Aether if needed for Tempest Trials or other similar endurance style events. Swordbreaker 3 cleans up his matchups against faster swordsman like Ryoma and Ayra preventing them from attacking back to activate their specials. Threaten Def 3 can be replaced with Threaten Atk 3 or Panic Ploy 3 if necessary to fight whatever threat you need. The +3 Speed Seal is a must to allow Ike to dodge a few characters that would otherwise be able to follow up on him. While this version of Ike is best to use on the defensive be wary that mages even green ones can take him out in 2 hits because of his terrible Res. He isn't really meant to fight mages so if possible keep him out of the way though Summoner Supporting him will help to fix this if desired. Overall this build should be able to keep up with most of the non magic blue part of the game as even lance users need to one shot Ike in order to really deal with him unless they want to get blown out by a fully powered Ignis/Aether.

Ike's optimal defense and his high attack will allow him to counter projectile users quickly. This will allow him to march into the front lines taking little to no damage against Bow Users, as well as take a hit against mages, to include the occasional Blue Mage that doesn't strike twice.

This build was used to finish Squad Assault 5, battle 5, placing Ike in the defense tile without moving. Keeping him covered against incoming Blue units, Ike was able to withstand hits from any and all Bow users and Red Sword users without being healed.

This build is an ideal counter against Bow Lyn, as Distant Defense (3) will allow him to shrug off arrows. Best to keep a green unit nearby to take out incoming Blue units.

This is a very enemy phase oriented Ike. At 34 SPD, it's very hard for Ike to double attack but it's also a bit difficult to double him as you will need 39 SPD in order to do so. People like Ryoma and Marth will not be able to kill him easily and his high attack will allow him to deal more damage.

Heavy Blade is taken as his attack is still high at 51 so he'll been able to use quite well. It's also allow him to charge Aether faster.

Vantage is core to this build. Often, you will find that you will put Ike in the range of people who are two ranged, primarily green mages and archers, as he has Distant Counter built into his weapon. Sometimes, multiple people can be in the same range and you can take a lot of damage. That's what HB+Vantage is for. You can charge Aether fast, possibly within on round of combat. And if you take a lot of damage and someone attacks you, you can counter before they have a chance to deal damage or finish you off. You can use Vantage 2 if you don't three readily available.

Threaten Attack is simply here to help out Heavy Blade. You can use Threaten Defense if you you want more damage.

Feel free to use an Attack Sacred Seal for more power and Heavy Blade Proving or A Speed one for more doubling and led getting doubled.

A classic Heavy Blade Ike build.
This explanation will mainly focus on Heavy Blade on Ike, but this can help those who don't really know how to build Ike in general.

An +ATK Ike is the best IV for a Heavy Blade build, due to HB relying on your ATK stat. -HP is probably the bane you'd want for Ike. -SPD is also pretty good, but you'll be doubled by a lot of strong units. -RES, like a SPD bane, is good but also risky since it opens you up for more damage against major blue mages like Linde, Delthea, or Thunder's Fist himself, Reinhardt.

Ragnell is here because of its amazing Might and great passive, allowing him to kill plenty of green mages such as Nino, Cecilia, etc.

Support skill doesn't really matter due to its flexible nature and will change with each team and build. For an infantry build, the best Support Skills would be Reposition, Draw Back, or Swap. Give him Ardent Sacrifice along with Renewal if you want him to double as a healer.

Special skills are where HB Ike shines. He can use any special skill within 2 turns or less. Ike naturally gets Aether in his build, which is a spectacular skill with a charge of 5. Another great special HB Ike can use is Bonfire or Ignis, due to his high DEF stats, giving him an extra 14-25 to your attack. Other great specials for Ike are Moonbow, Luna, Astra, Draconic Aura, Dragon Fang, or even Galeforce if you want him to have great player phase presence.

Heavy Blade for Passive A. Already said I'm using this one. Fury and Steady Breath is also great on Ike, but we're sticking with HB for now since he comes with it, and it works wonders on him.

Passive B is tricky to choose for Ike. It depends on what you want him to be on your team. Want him to be bait for ranged units? Give him Vantage. Want him to secure high damage on enemy phase? Give him Quick Riposte 2/3. Want him to be your main tank or a semi-healer? Give him Renewal. Desperation is possible on him, but not recommended for him. Swordbreaker can help him deal with Ryoma, Black Knight, or any other strong sword units.

Passive C is just as flexible, if not, more flexible than the Special skill slot for Ike. For a HB build, Threaten Atk 3 can help with securing Heavy Blade procs, or staying alive in a counterattack. Threaten Def 3 can also be useful on him, along with the many Hones, Ploys and Spurs. It depends on your team and/or Ike's role.

Sacred Seal can honestly be most of the available seals, but Attack + will most likely be the best since it helps proc Heavy Blade and gives him 3 more damage to deal. Brash Assault, Deflect Magic, or Distant Def could potentially work on Ike, too. If you want to cover a bane like RES or SPD, Sacred Seals can help patch that hole.