Series 5 and 6 are my favorite Doctor Who series, no contest, but series 1-7 are all good to great. I'd rank them: 5 > 6 > 4 > 1 > 3 (Counting the specials) > 2 > 7
After that things get...dicey,. Series 8 has huge problems and some really bad episodes. Series 9 has more good to great episodes then it has bad episodes but the few bad episodes are truly awful. Series 10 had a bit too much mediocrity mixed in with some ok episodes and a mostly great ending, and then Series 11 through 13 are just unwatchable for me except for literally three episodes, specifically Fugitive of the Judoon, the first episode of Flux (but only the first episode) and The Power of the Doctor. I'd rank these series
9 > 10 > 8 > 12 > 13 > 11
(with only series 9 and 10 being actually good, and 8 being mediocre but still a lot better then 11-13, in my opinion)
After that things get...dicey,. Series 8 has huge problems and some really bad episodes. Series 9 has more good to great episodes then it has bad episodes but the few bad episodes are truly awful. Series 10 had a bit too much mediocrity mixed in with some ok episodes and a mostly great ending, and then Series 11 through 13 are just unwatchable for me except for literally three episodes, specifically Fugitive of the Judoon, the first episode of Flux (but only the first episode) and The Power of the Doctor. I'd rank these series
9 > 10 > 8 > 12 > 13 > 11
(with only series 9 and 10 being actually good, and 8 being mediocre but still a lot better then 11-13, in my opinion)