Paladins are still very awesome! Ret specializes at dealing big bursts of damage while also bringing a ton of defensive utility for their group, including abilities like Blessing of Protection and Lay on Hands that can single handedly save a group from wiping. A Paladin's weakness is their relative lack of mobility, but they make up for it by being straightforward, hard hitting juggernauts that also pack a bunch of defensive tricks up their sleeve. I also played a Paladin in BC/Wrath, and I'm returning to the class for Shadowlands, so they definitely have my recommendation! :D
In terms of Covenants, all four are pretty well balanced for Paladins, so I recommend picking what you want. I believe Venthyr is technically the "top" DPS pick with a hard-hitting 4-minute cooldown ability, and they also come with a teleport ability to shore up Paladins' limited mobility. Kyrian is kind of meh for pure single target, but they're fantastic in any situation where there are multiple enemies, and they're always a strong pick for Holy. Necrolord got buffed recently, and they bring a solid and consistent damage damage increase; they're a good pick if you want an all-around ability and don't like the idea of Venthyr's 4 minute cooldown. Night Fae is the most controversial of the Covenants, with a suite of Blessings that allow you to buff yourself and your party members. Not everyone likes the idea of buffing their buddies, but I think they actually are more powerful than people give them credit for, and their Soulshape mobility spell shores up Paladins' greatest weakness (moving long distances).
So yeah, if you're looking for the "suggested" pick, it's either Venthyr or Kyrian. But all four Covenants are honestly really good for Paladin, so I'd encourage picking whatever you think will be the most fun for you personally! There aren't any wrong choices.
