Good Sentai Series For Beginners

As there have been so many series of Super Sentai, I'm looking for a recommendation of one to dip my toe into and watch through. I've watched a couple of episodes of Zyuranger (through curiosity, as I started as a Mighty Morphin' fan back in the day) and also a little bit of the Jet Man series. The one series that I have watched most of though is Tokusou Sentai Dekaranger, which I really enjoyed. This series seemed slightly darker and grittier than some of the earlier stuff I'd seen, and I liked that switch to a more at times, “dramatic” feel, for want of a better word.

So, are there any series comparable to SPD, with darkness and grittiness taking precedent over the more light hearted, tongue in cheek styles of other series? Heavy on the Sci Fi element too, like Dekaranger? Also, are there any cool stand alone Sentai films that fit this criteria?