Dr. Who is among my most favourite tv-shows ever (my most favourite is Buffy), but it is sometimes hard to tell how good it is, while it is going on. Episodes you didn't like at first often get better with repeat viewings. I suppose it has to do with that it's not always a simple show - some concepts can be hard to grasp on first viewings.
Some episodes I didn't fully 'get' at first now are favourites, like 'Hide', which I absolutely love.
"Doctor Who" Hide (TV Episode 2013) - IMDb
My favourite Dr. Who and companion are Tom Baker and Lalla Ward. Since the show restarted in 2005 my favourite doctor and companion are Matt Smith and Karen Gillan - although I love Billie Piper and Jenna Coleman a lot too.
I felt the Capaldi era started brilliantly with Deep Breath and I liked the first two seasons on the whole. So far I'm not so impressed with his third season though. Not so much to do with Pearl Mackie I think, who is very likeable, but the stories just aren't so strong, but I say it with great reservation though, as it's Dr. Who and, like I said, in retrospect episodes nearly always tend to get better with repeated viewings.
If indeed the quality of the show is going down, it's maybe a good thing there will be a new showrunner and Doctor - even though Moffat is responsible for my favourite era of the modern Dr. Who with the Smith/Gillan period.
I also think the title theme is the best of any tv-show ever. I remember as a child I even found that music scary and didn't dare watch. I finally watched it because it was on for the last time and I was too curious (it was the Tom Baker era most likely) and I loved it and regretted not having watched earlier.