Maingame...

Sully

Support (any mode) - Cavalier -> Paladin/GK -> Paladin/GK/WL/GR -> etc

Sully's promotion path depends on who you are supporting her with, since it changes what support bonuses you get. This is usually the build you roll with, as Sully is pretty tough to train early on and this helps reduce the amount of training you need to give her.

Lead (any mode) - Cavalier -> Wyvern Rider -> Wyvern Lord/Griffon Rider - > something that isn't swordmaster or assassin

Usually this is what you choose if you decide to play with male Robin, as that means Chrom has to go and find another marriage partner (as female Robin is usually Chrom's best wife), and you want to tuck Chrom into Sully. It's a rough ride in lunatic until she second seals into wyvern rider, but once she does she becomes fairly self-sufficient.

Stahl

Support (any mode) - Same as sully

as support units he's roughly the same as sully. His stats are slightly worse but when they're being support bots it's not really a big deal.

Lead (lunatic) - Cavalier -> Myrmidon -> Assassin -> Pally/GK

Stahl is a very tricky unit to make into a viable lead unit. If he doesn't second seal his EXP gains can cause him to not gain enough stats, but his reclass options are pretty mediocre. Myrmidon is actually decent earlygame because it helps buffer his speed and the avoid+10 and vantage skills are decent, but swordmaster/assassin are pretty awful for the midgame as the lack of 1-2 ranged weaponry is a big issue. It's bad enough to the point where it's more than reasonable to second seal at level 10 assassin to get to pally or GK ASAP and skill the level 15 skill, but that's up to you.