It occurs to me that I could do these list posts differently: instead of telling you what that the criterion is and then listing the plants that fit it, I could give you the list of plants and have you tell me the criterion. Basically "$10,000 Pyramid" for houseplants.
I won't do it that way, because it'd screw with anybody trying to google their way to a list of plants that had the criterion in question. But it'd be amusing. To me.
This one was unexpectedly difficult: I anticipated that I'd find circular leaves all over the place, but actually there are very few. Many otherwise-promising candidates actually have leaves which come to a point at the tip (there's a reason why we call it "leaf-shaped"), or are a little a little too stretched-out to call "nearly circular," or whatever. An even larger number among the succulents had almost round leaves, but with a thick petiole connecting them to the stem (like Crassula ovata or some of the Aeoniums and Echeverias), so the overall shape was more spoon-shaped than circular, and I didn't count them. But so here's what I came up with:
Perhaps to make up for the difficulty of finding a set of ten plants, the recommends are exceptionally easy for this list. Peperomia obtusifolia variegata, Saxifraga stolonifera, and Plectranthus verticillatus are all very easy and tolerant plants. Peperomia prefers somewhat warmer temperatures than the other two, and Plectranthus grows so quickly that it requires a little more maintenance, but they're all pretty easy, 2.0 or below on the PATSP difficulty scale.
The anti-recommend winds up being a tie between Calathea rotundifolia and Soleirolia soleirolii, both of which need a lot of moisture in the air and soil in order to grow well. Soleirolia works out well in terrariums, where it can spread out and form a mat. Calathea maybe would do well in a terrarium too, but it'd need a pretty big terrarium, as C. rotundifolia gets to be a big plant. Davesgarden.com recommends a spacing of 18-24 inches (46-61 cm) between C. rotundifolias when planted outdoors, suggesting that they can get at least that big across. Your average planted fish tank isn't going to be able to accommodate that.
Not pictured (please suggest others if you think of some):
Alluaudia procera, sort of (some specimens seem to have more of a tendency toward a teardrop or oval shape, but others have round leaves; this is maybe a difference in conditions, or possibly the plant is just naturally variable)
a few Begonia cvv. are kinda roundish, if not actually circular (especially the extremely spirally cultivars like B. rex-cultorum 'Escargot')
Breynia disticha 'Roseo-Picta'
Calathea makoyana, roseo-picta, and some other spp.
Ficus deltoidea, barely (often more triangular than round, but there seems to be a lot of variation between specimens, and some produce rounder leaves than others)
Ficus pumila, barely (usually more heart-shaped or oval)
Hoya kerrii, very occasionally (much stronger tendency to be heart-shaped, though sometimes leaves will have less of a notch at the tip)
Hoya obovata, most of the time
Lemmaphyllum microphyllum, sometimes (often more oval or leaf-shaped)
Maranta leuconeura kerchoviana, sometimes (usually more oval: younger plants tend to have rounder leaves)
Pellaea rotundifolia, frond leaflets
Pilea involucrata 'Norfolk'
Pilea microphylla? (leaves are nearly too small to really tell what shape they are)
Senecio rowleyanus (More spherical leaves than circular ones, but close enough.)
Tropaeolum majus (Though nasturtiums are not very often grown indoors, and they're super-prone to spider mites when they are.)