Magmavision2000

I cannot recommend the TOS and TNG BDs enough because they are absolutely incredible. There are a few places in TNG where they were not able to find the original film, so they had to upscale some footage in a few episodes, but both of them look absolutely beautiful overall and are highly recommended. Sadly, it seems that they might be difficult to obtain right now, though. As for which to start with... up to you, but personally, I say whichever one is actually in stock at your local Amazon or wherever else!

TOS is rough in places, especially if you watch it in its broadcast order (which is the order it plays in on the BDs, unfortunately), but it's much better in production order, which is how they wanted it to be shown before the studio or whoever decided to broadcast it out of order. The last time I watched it, I dumped the discs to my HDD and watched it in production order. It took up a lot of space and I had to delete each season to make room for the next one.

TNG sucks in the beginning but gets much better. It has some inconsistent writing (mainly Wesley and Riker) and a few boring characters, but it's overall good. It has more episodes than TOS, and more episodes total means both more good episodes and more bad episodes. The highs and lows of both of them are about the same, I think. I am ashamed to admit that I never cared for DS9 (probably because I was like 3 when it began) and never actually saw more than maybe 5 minutes of Voyager, though I am planning on getting the DVD sets. TNG BDs sold poorly and cost an insane amount of money to make, so there's basically no hope for DS9 or VOY BDs ever happening.

IMO, TOS and TNG are pretty worthwhile.  I'm still getting through all of TOS...There's also some fun to be had in each of the other series,  but I'm no expert.

I second that recommendation, TOS and TNG are well worth looking into.  In numerous places, you'll find "Top 10" lists for both series, so if you don't want to commit to watching every single episode, use these lists as a guide.  You should see the first six Trek movies as well, but in my opinion you'll appreciate them better if you watch them only *after* finishing TOS.

TNG took longer to "gel" as a series, so the first two seasons get a bad rap, but there are gems to be found there as well.  I'll recommend nine episodes from both seasons to help you steer clear of the dross.  From S1, check out the two-part pilot "Encounter at Farpoint", then "Where No One Has Gone Before," "The Big Goodbye," "Datalore," "11001001," "Home Soil," "Heart of Glory," "Skin of Evil," and "The Neutral Zone."  From S2, check out "The Child," "Elementary, Dear Data," "Samaritan Snare," "The Measure of a Man," "Contagion," "Time Squared," "Pen Pals," "Q Who," and "Peak Performance."  S3 on is pretty solid, with a few exceptions, though the four TNG movies were IMHO a disappointment.