At this point, the MCU has such a wide reach that half of the actors in Hollywood have been involved in it at some point. As it turns out, the MCU specifically shares a lot of talent with one of the other biggest tentpoles in nerd culture - Lord of the Rings.
This list will highlight all of the actors who have appeared in both the Marvel Cinematic Universe (sorry, X-Men doesn't count...at least not yet) and The Lord of the Rings (including The Hobbit trilogy). These actors have really cornered the market on being in beloved franchises, and being involved in both universes is kind of a cool checkmark to add to the resume.
Hugo Weaving
Weaving really shows his range going from Lord of the Rings to the MCU as he was able to so seamlessly move from good guy to bad guy. Obviously he had been in plenty of projects in-between Lord of the Rings and Captain America: The First Avenger, but Elrond was one of the roles he was most known for. He went from wise Elf-lord to Nazi scientist without it feeling like a strange leap, which is a testament to his skill. He always has a perfect balance of hamming it up and keeping a certain grounded energy to his characters, which is why he's one of the greats.
Cate Blanchett
Speaking of the transition from good to evil, none of these actors did it quite as extremely as Cate Blanchett did. Galadriel is kind of an embodiment of good in Lord of the Rings, from her white, glowing costume design to her intelligence and power as one of the oldest beings in Middle-earth. Her turn in the MCU was mythical in a different way, as she took on the role of Hela, the goddess of death. Her look was much darker and her personality much more full of attitude. Galadriel obviously has her own kind of power, but Hela is a bringer of destruction, and Blanchett can do both sides of that intense power so well.
Liv Tyler
Some people might forget that Liv Tyler was in the MCU at all, since her film is one of the most deliberately forgotten-about MCU films in the whole lineup. The Incredible Hulk has its issues, but Tyler's Betty Ross is one of the more interesting elements, and it's a shame that she never shows up in the MCU again. Unlike the previous entries, Arwen and Betty Ross seem to share more elements of their personality, but perhaps that comes down to Tyler's own soft-spoken voice and slightly ethereal demeanor.
Evangeline Lilly
Lilly is yet another female Elf character from Lord of the Rings who ended up getting cast as the love interest for a main hero in the MCU, which is an interesting occurrence to have happened twice. The original character of Tauriel created for the Hobbit movies was a fun addition, though it got bogged down by the character being shoved into the positon of love interest rather than an interesting character in her own right. This sort of happened to Hope van Dyne in the first Ant-Man movie before they elected to give her character more agency. In general, it seems that Lilly often gets relegated to that stoic female love interest role, despite her voicing her distaste for that very trope.
Benedict Cumberbatch
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Cumberbatch has many talents, ranging from his intricate hand motions as Doctor Steven Strange to being the motion capture reference for Smaug in the Hobbit movies. He throws all of his efforts into every role, and it's fun to see how he can be a dragon just as easily as he can be a Master of the Mystic Arts. His voice is one of his main tools, and it's fun to see him try out different styles, from a low dragon's roar to an (albeit occasionally shaky) American accent. The upcoming Doctor Strange in the Multiverse of Madness will certainly give him even more space to have fun with the character.
Andy Serkis
Speaking of actors known for their motion capture roles, Andy Serkis might be the biggest name in that category. His run as Gollum in both the Lord of the Rings and Hobbit movies was revolutionary and incredibly well done, cementing him as one of the actors who can best utilize motion capture in Hollywood. Of course, his talents go far beyond that, and he was able to create something memorable out of a role as minor as Ulysses Klaue in Black Panther, which is yet another testament to his skills.
Karl Urban
Another actor on this list who went from hero to antagonist is Karl Urban, who is known for playing Eomer in Lord of the Rings and Skurge in Thor: Ragnarok. These roles are so incredibly different that it's entertaining to compare them, but Urban is very capable of going from the classic fantasy hero/prince type to a much more seedy character as Hela's executioner in Ragnarok.
Richard Armitage
Armitage will probably be known to most as the Dwarf Thorin in the Hobbit trilogy, as his portrayal of the brooding king was one of the highlights of the series. Some might not even know that he was in the MCU at all, but he had a small role in Captain America: The First Avenger as Heinz Kruger, the Hydra agent who attacks during the initial Super Soldier Serum injection and ends up killing Dr. Erskine. He got to be the very first bad guy that a newly buff Captain America fights, which seems like a high honor in its own way.
Martin Freeman
He might have been playing a character known for being small, but Freeman's portrayal of Bilbo Baggins in The Hobbit trilogy made a massive impact. Many fans cite his as amongst the best casting in the trilogy, and it's true that he fully embodies Bilbo and his quirks perfectly. His MCU role as Everett Ross in Black Panther might not have been as flashy, but he still adds a great layer of personality and charm to the film.
Lee Pace
Lee Pace has appeared in two separate Marvel Studios, films playing the role of Ronan. The first time the character appeared was in Guardians of the Galaxy, where had taken a villainous turn and was working for Thanos. A younger version of the Kree warrior then appeared in Captain Marvel. In The Hobbit movies, Pace took on the role of elven king Thranduil, who is a morally complicated character and has just a little bit more depth than Ronan.