Blanket or generalized lists like this are difficult without enclosure-specific data and amplification ability that directly influences the system performance, availability, cost & preferred type of Bass!
32" - Top Pick
Peirce Audio 12k HT,
https://pierceaud.io/product/home-theater-series-12k-subwoofers/
24" - Top Pick
SI HS24, $1499.00,
https://stereointegrity.com/product/hs-24/
21" Ipal - Top Pick
Lavoce 216.00IP,$TBD,
http://www.lavocespeakers.com/single-product/?id=191
21" - Top Pick
Eminence NSW6021, $1149.00, 7yr warranty
https://eminence.com/products/nsw6021_6
data-bass.com
21" - 2nd Pick group
Lavoce SAN214.50/4ohm (new), $700.00,
http://www.lavocespeakers.com/single-product/?id=106
B&C's 21DS/21SW, $830.00/$949.00
This next evolution subwoofer uses a new, longer, four layer aluminum voice coil. The result is more energy in the gap, less heat, higher sensitivity, higher power handling, lower distortion and better overall performance. The 21DS115 features a 36 mm long, 4.5 inch diameter (116mm) Copper Clad...
www.bcspeakers.com
3400 W continuous program power capacity , 116 mm (4.5 in) split winding copper voice coil , 33 - 1000 Hz response , 97 dB sensitivity , 60 mm peak-to-peak excursion before damage , Neodymium magnet allows a very high force factor and linear excursion , Double silicone spider with optimized...
www.bcspeakers.com
* Measurements of the 21DS vs SAN214 show these drivers as all but equals. With B&C's new pricing pushing well within reach of the NSW I'd say Eminence, Lavoce then B&C
18" - Top Pick (There are so many)
SI HST18, $1299.00,
https://stereointegrity.com/product/hst-18-mkiii/
A few select JBL models, 2269, $1100.00 or so
Eminence NSW4018, $769.00,
https://eminence.com/products/nsw4018_8
Lavoce's SAF/SAN, $320.00/$430.00
B&C's 18DS115/18SW115, $730.00/ $714.00
This next evolution subwoofer uses a new, longer, four layer aluminum voice coil. The result is more energy in the gap, less heat, higher sensitivity, higher power handling, lower distortion and better overall performance. The 18DS115 features a 40mm long, 4.5 inch diameter (116mm) Copper Clad...
www.bcspeakers.com
3400 W continuous program power capacity, 116 mm (4.5 in) split winding copper voice coil, 35 - 1500 Hz response, 97 dB sensitivity, Neodymium magnet allows a very high force factor and linear excursion, Double silicone spider with optimized compliance, Ventilated voice coil gap for reduced...
www.bcspeakers.com
Fi has some nice models (the new HT line coming under a new company name)
Dayton has some nice models
18sound has a few, DD Audio, SI, Sundown......
* Dayton's VBSS PA460 (the 465 is disco), $170.00 was fun when it was $99.00 but at this price, the SAF184.03 makes more sense. There is a NEO PN470 at $250.00 that's interesting but again the SAF will beat it.
15" - Top Pick
SI HST15, Custom
SI SQL15, $369.00,
https://stereointegrity.com/product/sql-series/
Eminence LAB15, $330.00,
http://www.loudspeakerdatabase.com/Eminence/LAB15
Lavoce WAF154, $275.00,
http://www.lavocespeakers.com/single-product/?id=67
Dayton's HE-22, RSS HO, MX-15 (not the UM15-22)
Happy Reasearching!!