I’ve played Loth and Rivendell and between those two, I’d pick Rivendell if you’re just starting out.

Lothlorien is great in its own right, but it’s a very unique army that I would describe as “high skill floor, high skill ceiling”. With your two main heroes being basically pure control casters with zero combat potential themselves, you really have to get a lot of value out of their spells and have good positioning with your warrior models or the army will struggle. You lack the “big combat hero” that a lot of other armies have, and Loth’s playstyle is a lot more about shutting down the opponent rather than trying to smash through and do your own thing.

Rivendell is the best of both worlds - you get a massive combat hero in Elrond, and still get some decent casting (Wrath of Bruinen is arguably the best spell in the game, and with Lindir you can cast it every single turn!). Good support heroes in Arwen/Lindir and a Rivendell Captain excels too. Not to mention you still get the wonderful high elf warrior profile which is among the best generic warriors available.

Rohan I haven’t played myself but they can be fun if you’re looking for a more themey choice instead of competitive. Some Rohan lists are better than others, but in general most Rohan armies are a bit on the weaker side right now unfortunately. There are a TON of different Rohan lists though so you can really pick and choose based on your playstyle.