Considering only routes and not other factors such as tasks or casts:
TAR 1: Unique start in South Africa and a detour to Tunisia while in Europe.
TAR 2: The Southern hemisphere route is a classic. If the race had spent three legs in S. America as was originally planned, there wouldn't be so much route fatigue after Australia.
TAR 3: A lot of Europe after TAR 2, but the unique destinations (Mexico, Scotland, Portugal, Morocco, Switzerland) make this a surprisingly fresh route.
TAR 4: No Africa, but I liked how the route detoured a lot (French countryside - even though the leg was poorly designed, Kerala, Borneo, the jump to S. Korea and then down to Australia).
TAR 5: The classic six-continent, long-distance flight route. Good choice of locales. The only thing that really sticks out is the huge New Zealand detour.
TAR 6: Very similar to TAR 3. I still maintain that TAR 6 has the best destinations out of all the races so far (Iceland, Norway, Senegal, Corsica, Ethiopia, Sri Lanka, Xi'an).
TAR 7: Too much time spent in S. America. The destinations after that were pretty good though. I think the great circle route really throws off a lot of people.
TAR 8: No comment
TAR 9: This route was really good, a la TAR 5, but the transition from Asia to Australia to Thailand was done much better.
TAR 10: My most favorite 4-continent/westward route for the sheer diversity of countries: Mongolia, coastal Vietnam, Kuwait, the Indian Ocean countries, Finland, Kiev, Morocco, Spain.
TAR 11: Suffers from too much time spent in S. America and an awkward detour into Europe.
TAR 12: Another route similar to TAR 3/6 with some great destinations: Ireland, Burkina Faso, Dubrovnik, Italy countryside, Taiwan.
TAR 13: The Bolivia to New Zealand to Cambodia jumps are genius, but the route suffers by spending 4 legs in Delhi and Moscow. Had they moved two more legs to Western Europe or Africa, the jump to Portland wouldn't be so daunting.
TAR 14: Not a huge fan of 3-continent routes, even though Siberia was a nice detour.
TAR 15: Again, not a huge fan of 3-continent routes.
TAR 16: The zig-zag route itself is good. Even with French winecountry, Seychelles, Penang, & Singapore, it didn't live up to its potential, though.
TAR 17: Another route similar to TAR 12, with amazing destinations: English countryside, Ghana, Swedish Lapland, summertime St. Petersburg, Bangladesh, S. Korea. And the tasks/cast was great too.
TAR 18: I liked the Australia start and the big jumps in the route. Though most of the countries had been used before, the route explored newer unseen areas: Australian outback, rural Japan, inner China, Varanasi, the Matterhorn, summertime Rio de Janeiro. S. America at the end of the race nicely contrasts snowy Europe.
TAR 19: Very good route. Taiwan, Indonesia, Lake Malawi, Denmark, Belgium, Panama were all highlights.
TAR 20: Paraguay, Azerbaijan, and Hiroshima were nice detours. It really dragged in the second half though with the double legs in Tanzania and India though.
In general, the 4-continent Africa/Asia/Europe routes are my favorites because it feels that the race explores each continent quite in-depth. I do like how the producers have gotten more creative with their 5-continent routes lately, though, in the order of continents visited. Three-continent routes are really boring, in my opinion.