Tenchi Muyou! Ryououki

The same person designed both serieses. While El Hazard is not strictly a Harem anime like Tenchi is, there are still a lot of similarities between the two. 

Both involve a harem and comedy. The male characters are from a different type of families that they were unaware of and they are both skilled in sword combat. 

Men are from earth and hot sexy babes to fall into your laps are from space. Tenchi is older than To LOVE-Ru, but the themes are similar. Both MC's are clueless about the feelings of the aliens have for them and the heroines of both shows are of royalties too. Both fall for the protangist for unknown reasons and everyone (girls that is) in the universe wants in on the party. Lucky guys. 

Both have the male lead surrounded by beautiful girls who want to be with him. Both try to fight to help the female characters. Male characters are alike in all aspects. The only thing is separate is goddesses and extraterrestrials.  

The usual harem antics, although Tenchi features a bit of action. 

n-girls with powers after one normal guy.Two girls actively, and violently, competing for the guy. Also Tenchi is a great historical entry for the harem genre.  

Both have main female harem characters that are very likable and crave the general and sexual attention of the hero, also both have a likable main lead, they actually DO things to deserve the girls fawning over him compared to just getting ridiculously lucky. 

Both made by Masaki Kajishima....SERIOUSLY, fuse Tenchi Muyo and Evangelion and you have Dual Parallel, that's as blunt as it gets. 

Both animes involve a seemingly-average Japanese high school boy who stumbles upon at least one or more beautiful girls from outer space. Through this encounter the hero unlocks his own supernatural powers in order to protect his ever-growing household of family, friends, and prospective girlfriends (human and alien alike). Teen romance (and all its awkward funniness), love triangles, hotsprings, and sci-fi action for a good measure. 

Both are classic sci-fi romantic comedies starring an average Japanese high school guy. He inadverdently stumbles upon several beautiful (and often scanty-clad) female aliens, who become more interested in him than in their intergalatic adventures. 

Both the anime has the setting of a beast sealed by a sword which is released by the protagonist. 

Both shows are 90's harems, but Aa! Megumi-sama is a little more subtle about what it's trying to be. While Tenchi is more upfront and obvious about being a harem, Megumi-sama's main character by the end of episode one is already in a relationship. This goes against Tenchi's "Anyone could hook up with Tenchi and it's completely up in the air." Both shows have multiple women living under the same roof with a young man, and the girls are associated with aliens or have some kind of supernatural abilities.  

Both series have an agressive female character(Nyaruko-san for Haiyore! Nyaruko-san and Ryoko of Tenchi Muyo!) who lusts after the male lead and often makes unwanted advances towards their respective paramour. Both series are harems, although Tenchi seems to show more caring towards his than Mahiro does. 

Both are harem with a bunch of cute girls and awkward situations. The main heroes are quite similar since they both are living/have lived secluded lives in the mountains. The setup of girls are also very similar and in both series they have some kind of, more or less hidden, romantic feelings for the main character. Both of the series are also made by AIC which the gradual reviling of the more dark main plot shows. 

Made by the same company and has the same plots and story lines (romance). Instead of having 6 guys chasing 1 chick, there are 6 chicks chasing 1 guy. It involves magic and time travel as well as showing how fragile the human mind is. 

you want echhi comedy. here you are. both are harems and funny as hell 

Main relationship is just like the Tenchi-Ryoko-Ayeka triangle. Where it would be more Tenchi-Aeka than Tenchi-Ryoko based. Same feuds. Same humor. But its a hentai. 

It's a harem. It's got female aliens. Is there really anything else that needs to be said? 

I was pondering how Nymph looks so much like Sasami from Tenchi Muyo with her blue haired twin-tails, but that got me thinking of other resemblances to the classic anime harem: *SPOILERS* Tomoki = Tenchi (although Tomoki's a much bigger perv, both are heavily influenced by their grandfather) Sohara = Ryoko (childhood friend, first one to have a crush, could kill him with a single slash) / Kiyone (ambitious, gets upset at stupidity) Ikaros = Ryo-Ohki (pet-class, secretly powerful, adored by all, loves watermelon/carrots, can assume a spaceship form during combat) Mikako = Ayeka (purple-haired president/princess with the smug laugh) Suhara = Washu (eccentric scientist/explorer) Astraea = Mihoshi (ditzy blonde, though Mihoshi preceded the boob-service trend) Tomoko = Pretty Sammy (magical girl parody) Master of Synapse = Kagato (main bad guy) Daedalus = Achika Masaki (mom figure) Chaos = Yugi in Tenchi in Tokyo (little girl antagonist) Hiyori (movie) = Sakuya in Tenchi in Tokyo (later love interest revealed to be an alien) Harpies = Azaka and Kamidaze (alien guardians, ok that's a stretch) Synapse Monolith = Jurai Tree (icons from the other world) read more 

Both anime fall into the "harem comedy" category, and have some blatant similarities. Namely, the story centers around an average boy who is surrounded by girls of supernatural origin who fight for his affections. Rosario + Vampire's comedy is more sexual in nature when compared to Tenchi Muyo, but not by much. 

Both are old-school sci-fis and slapstick comedies. 

They both involve one guy getting multiple women interested in him while living in the same house.