Tenchi Forever! came out in 1999, and was the last work of the franchise until 2002's GXP and the 3rd OVA in 2003. While, nominally, it's in the TV continuity, it could easily fit into the OVA continuity as well. It earned the ire of a number of American fans for its tonal mismatch with the rest of the franchise. However, I think it's one of the strongest works in the franchise.
The main reason is that the characters, especially Tenchi, have some of their finest moments here. He's pissed off. He's had enough of Ryoko and Ayeka's years-long feud and its consequences. And after a welcome yet suspicious respite in Haruka's world, he's not content to just be strung along as usual, and wants to know just what the hell's going on. In short, it's a character examination and it turns Tenchi from mostly passive participant into an agent. It's almost a reverse School Days
, in the sense that it's a deconstruction of the harem romance setting's effects on the male lead, rather than on the heroines. [It may also be an apology for the execution of Tenchi in Tokyo as well...]
Ryoko and Ayeka also grow back some of the sanity they've been missing since the OV As
. You can feel their sense of loss and despair throughout, which pushes them to slowly develop a chemistry that partially repairs the group's overall relationship. The dynamics established by the end of the film certainly give this movie a greater sense of "finality" than the 3rd OVA did.
If I had one complaint, though, it's that this film focused only
on Ryoko, Ayeka and Tenchi. Mihoshi and Kiyone make a cameo, and Washu is mainly there to do the technobabble as usual. Sure, there's no question that Ayeka and Tenchi are the main girls throughout the franchise, but it still would have been nice to see a bit more out of the side characters.
An interesting change in this movie is the character design and costuming. The designs are, overall, less stylized and the eyes are especially different. I don't know if they're necessarily superior or inferior to the originals, but the change fits the tone of the film. The upgrade to late 90s apparel was not necessary, but still a pretty cool evolution.
I'm not sure this movie stands well on its own if you haven't seen the original series, but it's well worth watching if you're at least familiar with the material.