I like Voyager from CARETAKER to ENDGAME - basically everything about it. I hate FURY and I think that at the end of season six there is a bump in the road with a few episodes but basically I think this show is high quality from the first moment to the last. Out of its 172 episodes, approx. 160 are my favourite ones.
So no, I don't agree that the last two seasons are weaker than the previous ones but I don't think they are stronger, either. Every season had its moments, every season had a special atmosphere for me.
The first season has this magic of a ship lost in space, which gives the whole show a certain charm. Not only the premise but also the fact that our favourite characters have no idea where they are, how to get home, what dangers they are going to face, etc. There's a certain innocence about them, lasting into the second season, which in later seasons gets lost in the series.
The second season is special for me because there isn't a single episode here which I don't like. Yes, this includes THRESHOLD, I love that episode. Normally, each season has at least one episode that I like less than all the others but I like all of them in season two. Also, in this season we start getting to know our characters a bit better. Things are revealed about them from the past, Tuvok and Chakotay still figure heavily compared to later seasons. Characters are starting to know each other more, there are beautiful Janeway-Chakotay or Tom-Neelix moments and the second season amplifies that family feeling for me that was started in season one. In addition, we have the whole Seska-Michael Jonas story arc, which I think is interesting and basically the only story arc that was meant to be a story arc on Voyager.
The third season becomes interesting with the Nekrit Expanse. Voyager is, after all, to no small extent about exploration of space, just like other Star Trek shows. The Nekrit Expanse is even unknown to the inhabitants of the Delta Quadrant, so time for Voyager to explore. Memorable stories take place here with memorable aliens, like Marayna from ALTER EGO or the Mikhal travellers from DARKLING. Although the two main villains of the first two seasons are left behind (the Kazon and the Vidiians), the Nekrit Expanse more than makes up for this. Also, before they enter this part of the galaxy, we have FUTURE'S END, which somehow enhances the main theme for me - namely the quest for finding a way home. Also, I really love the Paxau holodeck programme and I wish we had seen it used in later seasons.
In season four, Seven of Nine is introduced and the tone of the show changes. This is the start of the "big borg show" and they will be recurring villains for the rest of the series. In contrast to a lot of people, who felt that they were overused, I think the Borg remain an interesting enemy throughout the show and every single episode with them is exciting.
Season four also introduces my most favourite villains, the Hirogen. This hunting race is intriguing and it's nice to know that even after THE KILLING GAME they will appear here and there.
As for the holodeck, my favourite programme is Leonardo da Vinci and I really like how he becomes a kind of role model for Janeway. Also, his workshop often serves as background for quite a lot of stories, strengthening the feeling that Voyager has "started a new life", so to speak. It's Renaissance time for our favourite ship and crew, a time to rethink the image and the atmosphere of the whole show. I can hardly imagine a better choice than Leonardo da Vinci for this.
This season is also really memorable for me because of the Tom-B'Elanna love story. This is the beginning of their relationship, with an amazing start while they are floating in space and as the show progresses, their relationship will become deeper and we are going to see several aspects of it. Their individual struggles, especially that of B'Elanna, will also become more interesting now that they have ach other.
Season five clearly shows that the focus has shifted: getting home is less emphasized here, although it isn't fully shoved in the background. Instead, we can definitely feel that the reintegration of Seven takes precedence and after her tense relationship with Janeway in the previous season, she has taken her first steps to really become part of the Voyager family. The show is focussing more and more on her and her relationship with Janeway and the Doctor. Some of the main characters, most notably Chakotay, get pushed in the background but it's not like they are not there at all, it's just that there's less emphasis on them. Tom and B'Elanna both get exciting adventures and so their relationship is further developed. In THIRTY DAYS, it is B'Elanna who gives Tom the final push to listen to what his conscience demands him to do and in NOTHING HUMAN it is Tom who argues that B'Elanna's life should be saved no matter what ethical dilemmas this will involve. There are further great character moments for Janeway in COUNTERPOINT, for Tuvok in GRAVITY and for Harry and Chakotay in TIMELESS.
This season also introduces the last of the recurring villains, the Mallon, who are in my opinion the weakest enemies on the entire show.
Season six, while further focusses on the themes that I've mentioned so far, introduces some of the nicest aspects of exploring space. ONE SMALL STEP and BLINK OF AN EYE are both great episodes on their own, both relating to what Star Trek is about: exploring space. In contrast to a lot of people, I think the two FAIR HAVEN episodes are just great - it's nice to see that the family feeling that the Voyager crew radiate is extended towards people made of photons and forcefields. Voyager, after all, is in no small part about holographic rights and it's good to see that the show does not only do lip service to this but shows that flesh and blood people are capable of welcoming "artificial" people into their big family.
But the greatest thing to me about season six is that the main topic of the show - getting home - is given a fresh start with PATHFINDER and everything else that follows from this episode. Regular contact with Starfleet will give an extra dimension to the show, partly because some beloved characters from other shows are being brought back. Barclay and Troi are both very good in their roles but so are Tommyboy's father and Doctor Zimmermann. Again, seeing that some parts of the stories take place back in the Alpha Quadrant somehow enhances the feeling that this is a Star Trek show with a lot of good elements that ST has in general.
Season seven is a bit of everything, I think. This is the culmination of the show connecting the first three and the second four seasons and connecting the Delta and the Alpha Quadrants. First, people are being brought back from the first three seasons: Q, Seska, Carey. Second, the theme of getting home is enhanced with stories partly taking place in the AQ and with the fact that there's regular contact with Starfleet now.
While the Seven-Janeway-Doctor triumvirate keeps being in the foreground, B'Elanna and Tom are also focussed on more than in earlier seasons, giving a prominence to these two characters that we haven't seen previously. Altogether, this season focusses more evenly on the characters than the previous ones, on both main and recurring ones, with even Tuvok getting a long-awaited ponfarr (sp.?). Speaking of Tuvok, even the Maquis theme is brought back, further connecting the show with other Star Trek shows.
To me, the real strength of this season lies in some of the stories - actually in almost all of the stories. I've already mentioned Tom and B'Elanna, who will become married and ultimately parents. The Doctor is shown in his struggle for holographic rights again, in addition to being shown in his professional quality as a doctor in CRICAL CARE. Or think of the emotional farewell we get for Neelix - a truly unforgettable episode. Also, all the two-parters turn out to be good ensemble shows, and although I'm a bit less keen on ENDGAME, it is best at what Voyager has always been best at: family.
So no, all in all I don't think that the quality of the show deteriorates in the last two seasons. Also, I'm not an expert on DS9 but if memory serves, the final season was very good there, too. So there are at least two shows where I don't think the statement that the last season(s) are worse than the previous ones holds up.
Having said that if there is something that I really miss from the last two season is a recurring enemy (in addition to the borg, of course).