What would be the best build to play for a 2-people group? One character is a polearm paladin going vengeance. Since he'll be weak on range attacks and AoEs, I figure maybe I should play those strengths, but I'm really not sure. Maybe it should also be a good healer...? Or maybe I need arcane? I really have no clue, I'm up for anything!
Personally, I'd go Rogue 2/Bladesinger X in that situation (if your DM allows Bladesinger) if I could scrape up decent stats for it. You'll be a skillmonkey, a capable ranged attacker, a grappler when necessary, a backup melee tank (especially if you take Mobile and Booming Blade), the party scout, and the party wizard (intel specialist/summoner/Counterspeller/etc.). Essentially you're the Rogue/Wizard and he's the Fighter/Cleric, two halves of the classic four-man party.
Although Bladesinger is not mandatory. Given how good Cunning Action is in the dark, you have a wide latitude to try a different specialty like Abjuror or Necromancer instead, if it sounds fun to you. I picked Bladesinger because it's a nice generalist specialty which adds some movement and defense options to free your paladin up from having to worry about your safety.
SailorNash's suggestion of a Moon Druid is also good and would be my second choice, but I like the wizard spell list slightly better.
Any specific bard or druid builds? I'm open to either one, but I know next to nothing about them. I'll check out the guides, but any pointers would be helpful
One nice thing about Moon Druids is that they really don't require any "builds", they just work right out of the box. But if you want a specific suggestion, I'd say to get a good Con and a good Wisdom; take a variant human with the Mobile feat (and Thorn Whip cantrip, which works with Mobile and can be used in human form to protect the paladin without endangering the druid) and plan on taking Sentinel (to protect the paladin/do extra damage in Wildshape) and Resilient (Con) (so your spells don't go down in wildshape form) fairly soonish; get to know your wildshape forms including giant hyena, rhinoceros, and giant constrictor snake; and get to know your spell list including Entangle, Goodberry, Spike Growth, Barkskin*, Conjure Animals, Polymorph (into a Giant Ape or a T-Rex), and Conjure Elemental/Planar Binding. (Will need help from the Paladin to cast Magic Circle prior to any Planar Binding shenanigans with elementals though.) Between your twice-per-short rest wildshape forms and your per-long-rest spell list you should be able to kill anything you can't run from, and run from anything you can't kill.
Edit: BTW, I recently heard someone suggest Savage Attacker for a Moon Druid, and I have to admit that that feat would be less useless to a giant crocodile or a rhinoceros than to a mere human being, especially when combined with Sentinel (because it's once per turn, not once per round). My informal test (rolling ten Giant Crocodile hits for 3d10+5 each, with Savage Attacker on, twice) produced gains from Savage Attacker of 6 HP per round the first time and 1.8 HP per round the second time, so it's a DPR boost probably somewhere between +8% and +29%, to one of your two attacks. That's actually not terrible, if you have a feat to spare.
-Max
* Actually I'm not a huge fan of Barkskin normally, since Mage Armor is so much better for a Moon Druid (longer duration, no concentration, almost the same AC for many animals, only a 1st level spell) but you have no friendly wizard to do the casting. If you can get the paladin to take a level or two in Sorcerer then he can cast Mage Armor and you can cast Fog Cloud and then turn into an Anaconda (Giant Constrictor Snake) or Earth Elemental, with blindsense and tremorsense respectively, so that you get advantage to attack almost all your enemies and they have disadvantage to attack you back. But that doesn't work if you're spending your concentration on Barkskin.
