It really depends on what you want to achieve. In terms of PvP I can't give too much advice because I stopped playing PVP from chapter 5 onwards.
The following are just suggestions on what class combinations are really common because they are very good in their respective roles:
Tank? Knight/Warrior is the best tank in terms of group support, but you can also play Champ/Rogue or Warden/Warrior if it's just for having aggro (I guess Champ/Priest is also used because of it's 20% dmg boost? (someone might correct me if wrong).
Burst? Scout/Warden, Scout/Rogue, Rogue/Mage, Rogue/Warden, Warrior/Scout Maybe Warlock/Champ(requires a lot of set farming tho first) , Warden/Warrior, Knight/Mage with magical 2-h Hammer + MDD gear, maybe Mage/Warrior(Requires a 2nd equip with str/sta 6 stats on the items and still is only decent but not top tier basically), Mage/Warden if you have A LOT of MDD support.
-> Just heard but never seen, but it seems that Warden/Mage can also deal insane amounts of dmg.
Sustained(or rather "long term") DPS? Warrior/Mage. Champ/Rogue. But also Rogue/Mage, Warden/Scout in really long fights(even though the W/S is better as a supportive PDD).
AOE? Champ/Rogue, Scout/Mage, Mage in general(probably some more classes).
Healer? The Druid is the meta healer because of support in terms of def reduction on bosses.
Druid/Rogue, Druid/Warrior are most common because of even more PDD Support. But sometimes a Druid/Warden can be good too, Priest/Warlock and Priest/Rogue can be played too even if they are missing the def reduction.
Support? Warlock/Mage(Which is also the class which requires the least amount of equip to be really, really impactful because of the insane amount of buffing/debuffing). Mage/Warlock(Needs some equip but gathering some crit rate should seal the deal). Warden/Scout is also really good because of DMG and DPS buffs.
Solo farming cards/mats etc? Warrior/Rogue because it permanently has +18% move speed or Rogue/Scout because it can have 3 drop runes and has an additional 20% drop rate skill.
I'm pretty sure I forgot important combinations, but these combos are good in their roles but you can obviously also try out stuff and read online to see what the elite skills of certain class combinations do so you might wanna check that out too: