I don't have any set scale for rating bases. Each is judged based on the figure it's attached to, the company that made it, and the presence it gives off. Some examples of positive bases are:
Tsukihi's base item #162017: Rated 9/10
Probably one of my top favorites, and the reason I dropped a lot aftermarket to get her. The bright red color and repetitive pattern made by the latticework draws the eyes to the figure, but it's still simple enough that it doesn't take away from Tsukihi herself. The straight vertical and horizontal lines are a nice clash against the way she is leaning at a diagonal angle, and the thinness of the poles accentuates the thinness of Tsukihi herself.
(Honestly pretty much every Monogatari figure has an amazing base)
FunnyKnights does a decent job on their bases for the KanColle line (mfc link): Rated 6.5/10
The simple wood design with a placard of the ship's name gives off the feeling of an actual ship model, which is perfect for the atmosphere. The problem I have with a few of them though is if you put the placard in front, the figure looks off center and at a bad viewing angle. They could be a little smaller in diameter as well.
Saber Alter item #287689: Rated 8/10
An amazing looking base, fits the atmosphere of the figure perfectly. I'm usually not a fan of asymmetrical things, so sometimes I get a little twitch in my eye when I look at it, and sometimes I wish it didn't stick out so far, but when looking at the figure as a whole everything fits wonderfully.
Homu & Mado item #397001: Rated 7/10
A lot of people hate this base. I personally love it. It's simple, yet has a presence that makes it stand out on a shelf without taking away from the figures by being so tall, like a built in riser. I also love the fact it reminds me of a wedding cake topper, which adds so much to the atmosphere of the two little lovers.
Laura & Charlotte item #371718: Rated 8/10
Heavy duty, a folding support strut, simple design. It was clearly made for functionality instead of being flashy, but that's purrrfect when you already have 2 figures all cute and cuddly together in one small space to focus on.
Last of the positive bases I'll point out are the opaque bases of Zuihou item #198424 and Wokyuu item #198412: Rated 10/10
I love these bases. On their own they look amazing, creating a nice atmosphere but not taking away from the figures. But, the best part about them is when you get fancy and add lights underneath like I did, they take on a whole new level. (mfc link) Pretty damn amazing in my opinion!
As for negative bases:
Mikasa item #198359: Rated 4/10
In terms of atmosphere it does hit the mark, but it's way too unstable and top heavy. I've had many issues with it falling over. The entire bottom with the stabilizing weight fell out and I had to glue it back in, and the bottom isn't perfectly flat so it wobbles (even before it fell apart) Plus it's a bit too tall for my liking.
Kanade item #105081 and Sagiri item #397042: Rated 1/10
I am so disappointed in these, especially coming from GSC. The characters are somewhat basic already, so I think a little flare would have helped them stand out better.
Makise item #35297: Rated 3/10
I like the gears she's sitting on, but the main base is just plain and clear with a gear molded into it and it has a weird shape on the bottom, almost like it should fit into something. Not only that, but the support under her boot is white. Why not make that clear too? It's so bad I ended up putting her on top of my bookcase, so I can only see her from the knee up.
The DesDiv6 girls item #286436 item #331562 item #286431 item #331565: Rated 3/10
A good concept, having 4 figures with adjustable bases that can be combined. Poor execution. Each individual base is actually a few different pieces, so they can be rearranged, but the slots and tabs are placed in weird spots, so you're actually extremely limited in combinations. They should have made them universal and symmetrical. Not only that, but once they are all combined together, Inazuma and Ikazuchi barely fit. I've tried so many combinations to give them the appropriate space, but nothing works.