See title. There are favorite leaders, then there are favorite second-in-command, even third-in-command. Which legends do you find to be your favored subordinate in deck making?
Grenzo, Havoc Raiser - My go to 2nd/3rd in command when I build any creature deck involving red. I just love the versatility it grants (beyond goblins), allowing red decks to manipulate battlefield and cover its behind with cards of other colors. I use him in all my red, Boros, and Jeskai decks with token generation. The same could be said about The Locust God in any deck with U/R, needless to say the two work wonderfully together.
Haakon, Stromgald Scourge - 2nd-in-command for my Varina deck, to be a zombie, a sac target, and casting Knight/Tribal spells from graveyard (often came from Varina's discard). God-Eternal Oketra and The Scarab God are also powerful allies but they're usually not my tutor target since Haakon can do more control with your own graveyard.
Ephara, God of the Polis - I enjoy creature-based strategies (as you might've noticed) so her ability to bring longevity to war is a treasure.
God-Eternal Kefnet - Leader worthy, but also easily abusable in command zone, so I prefer using him as a 99 in spell focused decks.
The tutoring commanders in your 99 are often overlooked. The thing is that you only need to activate them a couple of times to get value. People think that unless you specifically have millions of cards they can target, they are not good. Captain Sisay..ba.ba.broken. Zur the Enchanter is often not thought about as part of your 99 if you didn't end up playing him as your general. Remember you don't actually need that many targets.
I guess Xantcha, Sleeper Agentin Nekusar. I added her quite recently and she has done so much damage and impact on the game the few times she resolved. She does what I want her to do. Take some focus off of me, gets the game going and she can be a 1 for X. One game she did like 30 damage. And she isn't my wincon or anything, she's one in the 99.
Others that are more niche that I use in the 99 of my decks that may not have places in most other decks: Surrak Dragonclaw - Decent sized body, makes my creatures uncounterable and serves as one of several trample enablers for my Maelstrom Wanderer deck. Seizan, Perverter of Truth - Helps out my Nekusar deck. Niv-Mizzet, the Firemind - Along with Curiosity and Psychosis Crawler, it is an alternate win-condition of sorts. I can also combine it with Notion Thief if I don't want to kill my opponents with their own cards, though doing so is very situational and I'd only do it if I'm not hurting myself with my own Spiteful Visions.
Varina doesn’t really have a lieutenant per se. Ghoulcaller Gisa is in there but I haven’t playtested it enough to know if she’s reliable enough to be 2IC. God-Eternal Oketra is in there too, but again, early days. Realistically, the true MVP of Varina's deck is Tombstone Stairwell - that card is bonkers.