+1 to Bass Bone. Makes good sense. Each instrument model (and to a lesser extent, each separate instrument) has its own character, but the dominant character in the interaction is that of the player. The thing is to work out what set of sounds you are trying to make, and to then find the instrumental set-up that makes it easiest for you to make those sounds.
The 'hard machine' (i.e. the bone) is more reliable in response to change than the 'soft machine' (i.e. the player), and so it becomes tempting to fiddle endlessly with one's equipment rather than take the longer but more difficult route of sorting problems out by practising. In reality of course, most of us give in to the temptation at some point or another... And using the wrong instrument can really obstruct your development as a player - if you are fighting your equipment rather than playing it, then things will obviously feel a lot more difficult.
These days I personally play a Rath R9, which is not a complete description in itself. A decent way to think about the importance of the components that go into a modular trombone is to rank things in decreasing importance through the instrument from player to room - 1) mouthpiece, 2) leadpipe, 3) slide bore, 4) valves/gooseneck, 5) bell flare. In a distant last, place any effect of metal composition change. This is a simplification, but is quite a useful way to think about it. Many of us have a mouthpiece that we already get on particularly well with, so will after all that treat that as a fixed parameter anyhow. The leadpipe is a vastly underrated part of the puzzle; getting this right has almost as large an effect as getting the mouthpiece right. I'm personally at the moment trying a non-Rath leadpipe in mine, and am feeling positive about the effect so far - it's been selected as offering quick low register articulations while taking plenty of air happily. Slide bore - single .562" vs dual .562/.578 is the choice really, although Shires do offer an "oversized" .578 single slide option. It's my experience that anything larger than .562 single is too woofy for me to use, but some love dual bores, which tend to match well to very big mouthpieces and players with very large airflows. Valves - how much blowing resistance do you like? "As little as possible" is not as good an answer as many people think... If you are well used to playing on rotors, more open designs can throw you off your stride in unexpected ways (e.g. articulations - part of the reason for the leadpipe change I mentioned - essentially compensating for resistance lost elsewhere in the tube). Bell flare - bigger makes for greater directed volume at the cost of some loss of focus in the sound. 9.5" is the standard; 10" and 10.5" less popular variants. More and less open bell throats have an effect too (and, tracing back, so of course does a different bore profile through the valve area, in a similar fashion). It is often very hard for the player of the instrument to judge the exact effect of a bell change - it's more to do with the listener than the player.
So modular bones, while they offer exciting opportunities for fine-tuning, do require detailed thinking about how the different aspects interrelate, which can prove endlessly confusing to players. I would not suggest buying a modular trombone until a player is clear in their mind on exactly what they want from their playing.
There are various makers and models out there from the last half century or so that have their devoted followings amongst serious players. Among them are (in alphabetical order):
Bach:
Historically, one model, 50B, with variants with various rotor layouts and wraps, larger bell sizes, bell metals. More recently, also available with Hagmanns or Thayers. Used to be popular in bands, not so much these days. New models are priced astronomically for what they are. A good solid choice that will shout and cut through a band, but unsubtle in tone.
Conn:
Made the original modern bass trombones, with some 1920s instruments still very usable in modern ensembles. Various models made at the Elkhart plant (1920s-early 70s) - 70H (original model, up to 1955, single rotor, TIS, a lot of variety in specimens), 72H (1955-1968, standardised 70H, TIB), 71H/73H (1968-1979, single/double dependent rotors, 72H with valve layout remodelled), 60H/62H (1968-1972, single/double dependent rotors, 83H (briefly made early 80s(?), double independent rotor 73H version), 110H/111H/112H (80s replacement line for the older basses, not many are fond), 62H/62HCL/62HG (modern ("Gen II") take on the Elkhart 62H, offered also with Lindberg and Greenhoe valves).
As you might guess, I am quite a fan of the colourful sound that comes from older Conn bass trombones, but have reluctantly concluded that their blow just does not suit my playing style. The Elkhart 62H has a legendary status among this bunch - the modern 62H is not the same instrument at all to play or listen to. These are instruments that aren't in their natural habitat shouting in the low register in a brass band - they can be made to do that very nicely, but it does require your playing to be in tip-top shape. The older instruments tend not to react well to large mouthpieces.
Edwards:
As with Rath, modular, wide variety of options on the basic model, B454, which was the first on the market to use this approach. To my taste, though beautifully manufactured and easy to play, they tend to sound a bit bland. But I know plenty of very good band players love them.
Holton:
A funny one... Holton had it so so right in the 1960s with their TR169 model, which plays like a dream to anyone who has taken the time to learn to deal with the quirks offered by the later TR181. Very few of them around, so if you see one, grab it and cherish it. They were single-rotor, but the drop-in second valve option worked very well. The TR181 is popular in bands and well-suited to the medium, but those of us who've spent time of them will know their playing faults - a reluctance to speak at piano and a dull sound generally in the lower dynamics; some criticise the valve blow also - I personally like it.
King:
Another funny one. The "Duo Gravis"/6B model hasn't been made for years, but offers something that nothing else that has ever been on the market that I'm aware of has - a .562 bore through the valve section. This makes for a naturally very focussed sound, and they are great for cutting through anything, with well-designed rotors and a well-matched leadpipe. King replaced with the 7B (which had some popularity) and 8B (which didn't really) models, and no longer manufactures bass trombones (as with Bach, Conn and Holton, King have been subsumed into the giant Steinway/UMI manufacturing group). Those who like the Duo Gravis tend to really like it.
Rath:
As above, models R8 and R9 (really the same thing - 8 denotes nothing more than single valve).
Shires:
As with Edwards and Rath, modular. Not often seen in bands, but I think they'd work well. As with Edwards, the ones I've heard have tended to seem a bit bland in sound quality to my ears.
Thein:
Super-pricy German modular make. Lovely bones, hardly ever seen due to cost. Maybe a bit too much of a dark sound for banding, but I've only ever tried one, which is hardly a representative sample.
Any of these (and more that I've omitted through forgetfulness or lack of experience) could be the "best" bass trombone for some particular player.