Tsukuba because it is a great place to drift and a great place to test a cars natural inherent "balance" with the fast sweepers combined with tight hairpins and a few tricky "bumps" on the racing line. A great technical track which rewards cars with power that can be put down effectively out of the bends. I just wish they had the bike track with the added chicane so we could use that variant for Kart racing.
It also gives some cracking, fun and close online shuffle races. Tsukuba in a RWD on comforts is just great fun! I can see it would have limited appeal to racing tyre fans though.
Fuji because it's a challenging track to drive and to tune for, a long straight combined with low speed twisty sections and some 1st and 2nd gear corners. Blind (multi) apices, odd and challengine cambers and corners that test both high and low speed cornering. I also like the fact that many of the corners are "linked" and poor exit from one will lead to poor entry into the next. It is a track that requires some planning and forethought during a race if you want to pass and not be passed straight back at the next corner (or in the case of the final corner, bleeding away exit speed will mean poor performance down the straight).
Fuji is a track that is easy to learn but very difficult to master.
I have a few reasons to hate Fuji, where to begin...
1. The track is boring, visually it's dead and dull so it makes me sleepy (not really but you get the point)
2. Elevation differences are very small but on such spots that you have to be extremly careful with the throttle while going out of the corners (especially when your tires are low/endurance racing)
3. 3rd sector, call me dumb but I just never could work out a line there. I'm not slow but I just can't work out a line through T3
4. Last corner (need I say more?)
5. Personal reason. 2 out of my 3 online races were on Fuji, both were enduros. I spent over 10 hours practicing there and I couldn't work out the line in T3. Oh, and I failed both times - the track is cursed
I'm bad on Fuji, the track is not flowing well and it's dull - and that's why I hate it.