As their dodge chances are quite high, it is for this reason that people prefer to simply take a Bounty Hunter or Occultist so that they can deal with the backline easily. However, as an alternative, if you have a way to deal with the opposing backline outside of pulling them to the front and beating them with your own frontline, you can do something like run a Plague Doctor with the ability to stun the backline and hope that they don't dodge. For this reason, taking a Bounty Hunter or an Occultist to use their skills that allow them to pull backline enemies to the frontline is usually a good idea. Many enemies that are in the backline are very fast and can cause your team to have a high stress count if you don't kill them fast enough.

Having at least 1 backliner that pulls or a way to deal with the opposing backline.

What you do with these frontliners is also very fluid: You can have a team centered around buffing a Leper so that his low ACC isn't much of an issue, or a build centered around stunning the entire enemy team with a frontline Hellion and a backline Plague Doctor. Generally, however, this will be either a Hellion, Highwayman, or Leper, due to their very high damage output. This one is a little bit more fluid, as you can have many different classes take up this role.

Having at least 1 dedicated frontliner that deals damage to enemies'.

This doesn't mean your party needs a healer, however it is highly recommended you take one for regular parties.

The preference on taking which one depends on the person, as Occultist has more utility while Vestals have a higher chance to actually recover the health of your party. This will usually be a Vestal or an Occultist, as their backline capabilities are greater than that of a Crusader or Leper, and their heals can recover the health of your party by a much higher amount.

Having at least 1 dedicated backline that heals the party.There are 3 different factors that usually make up a good team: This page is dedicated to party compositions that work well in Darkest Dungeon.

Darkest dungeon team comps