Speaking for myself, the first season ova is great. But past that I prefer Universe. I think Universe understands what it is a bit more and is this more effective. The later ova continuity gets bogged down real quick with things not pertaining to what makes tenchi work as a concept.
Like look, it's a group of space girls, they live with this guy, and they have cute space adventures. Please don't beleaguer that.
I absolutely love it.
The characters, the relationships between, the worldbuilding.
I wish more of the doujins were translated just so I could know more about Geminar, or how Jurai works with the Kuramitsus and the GXP.
It's like WWII.
It was a horrible thing, but good has come of it.
The war saved America and Germany among other countries from depression and advanced technology greatly.
Ai! TM showed there was still interest in the franchise, and the porn should be good too.
Start with OVA 1+2.
OVA 3 and GXP are happening at about the same time.
Isekai no Seikishi Monogatari happens 16 years or so after OVA 3.
Dual! Parallel Trouble Adventure is canon, but only tangently related to the rest. It's highly recommended, watch it after OVA 2.
Universe and the movies are separate from the above. The characterization is weak, but they're good.
Tokyo, Ai, and Pretty Sammy are to be avoided.
The creator also releases doujins with stuff he hasn't been able to animate or work into the show yet. Background info and worldbuilding.