I love flats. They sit so much better in the mix than rounds. When I started playing bass I played roundwounds exclusively for the first decade or so. Then I started using flats from time to time. Three decades later it's 80% flats and 20% rounds. It seems to me that lots of people regard flats as quirky "niche" string that only work for certain genres. I used to think that way but I've realised it's almost the opposite: Rounds are actually less versatile than flats IMO. In fact, some of the brighter rounds that I used to love sound grotesque to me now and I just end up wasting time to get rid of that metallic clank in the mix. Flats are come pre-mixed. Well kinda...
My favourite flats:
1) Thomastik Jazz Flats: Lovely, organic sound. Very modern and sorta, well, jazzy. More upper mid bite than most flats but not metallic/clanky/zingy at all. Someone described them as having an orchestral vibe and that pretty much sums them up for me. There's definitely a hint of upright bass there (but also lots of sustain, not thumpy). These strings shine on a piezo equipped hollowbody fretless. But they sound great on pretty much ANY bass. They are extremely versatile. They are very low tension which might take some time to adapt to but defintiely worth the effort. I love them! I only like the long scale set though. The short scale set has a MUCH thicker E but same gauges for the rest of the strings. This causes a HUGE jump in tension between E and A on the short scale set. So much that I find them too uncomfortable to play. They sound just as great as the long scale set but are FAR too unbalanced IMO.
2) Pyramid Gold: The E-string is INSANE. The first time I tried these strings I thought I had gotten a faulty E-string. It's totally "dead" out of the box while the rest of the strings are actually quite bright and take some time to break in (especially the G). But the E-string is "dead" by design and once the rest of the strings mellowed out I soon realised that these strings have the FATTEST sounding E-string on the planet! The long scale set is amazing (but the D-string vibrates with such a huge amplitude that it may require some basses to be set up with more relief and higher action than normal to be buzz free). The short scale set is even better! Instant 60s vibe but great for lots of other stuff too. The bouncy sound/feel and the thunderous E-string you get from a set of broken-in Pyramid Golds is truly unique. I love them! And of course, a Höfner violin bass won't sound like a Höfner without Pyramids. That alone secures the Pyramid Gold's place in the history of rock. Possibly THE most famous bass sound EVER. They are very fat/dark though and may get muddy on some basses. On a dark sounding P-bass (such as one equipped with the extremely dark sounding Fender Pure Vintage 63 pickup) the Pyramid Golds are a bit too dark IMO. But on a brighter bass, or even a brighter P-bass like the Yamaha BBs the Pyramids Golds are sweeeeeeet!!!!
3) La Bella DTFs: Nothing beats these strings for that old school, thumpy soul vibe. The Jamerson set is too heavy for me but I love the lighter gauges and feel naked without at least one bass equipped with DTFs at all times. Very thumpy if you want them to be but they can acually have a surprising amount of sustain if you use them without a foam mute. They do sound great with a foam mute though. In fact, they sound mindblowingly amazing with a foam mute! I love these strings! Just the right amount of mids. Fat lows. Smooth, laid back highs. One weird thing about them: Often they sound buttery smooth out of the box but sometimes they can be very clanky/metallic and need to be broken in before they are usable. Contrary to popular belief they actually break in quite fast though. Much faster than many other "old school" flats (like the GHS Precision flats that take FOREVER). But I've never experienced such inconsistency out of the box from any other strings. One set: smooth. Next set: CLANK. They sound amazing once they're broken in though and they've got so much character that they will make pretty much any bass sound like LaBellas rather than a P or a J or an active coffee table bass (which may be good or bad depending on the situation I guess...).
4) Rotosound 77: The sound of British 70s rock. Not thumpy at all and not particularly fat lows but those mids... Possibly the most lovely mids that I've heard from a bass string EVER. They are very bright and clanky out of the box and the fans of these strings are divided in two completely separate groups that probably aren't on speaking terms: The ones that love the bright, fresh, almost roundwound'ish sound and complain that they don't last very long and the ones that love their smooth broken in sound and complain that it takes forever to break them in (particularly the E-string that stays metallic and clanky for MONTHS after the others have mellowed out). I'm in the latter category. I love these strings for their lovely, smooth broken in sound. They're not fat or thumpy. But they've got amazing mids, just the right amount of presence and are perfectly balanced from highs to lows. I can only handle the lightest gauge though (40-100). The heavier ones KILL my hands for longer sessions. Anyone who thinks the LaBellas are bridge cables should try the heaviest gauge Roto 77s (but don't blame me when the physiotherapy bills start adding up!). The short scale Roto 77 set is unfortunately close to useless. The extremely low gauge makes the E-string so floppy that it sounds out of tune. Which is a HUGE shame as the SOUND of these strings is a match made in heaven for many short scale basses.
That MY favourites. My PERSONAL favourites. IMO. YMMV.
Flats that I don't like very much:
DR Legends (horrible IMO)
Chromes and similarly metallic/clanky flats (I prefer rounds for clank/zing)
GHS Precision flats (just a bit uninspiring and dull IMO, like a poorer version of LaBellas that take a lot longer to break in)
Flats that I haven't tried but I'm curious about:
Optima RB: Classics, never tried them, want to BADLY!!!
Optima Chrome: Got a set last week but UPS lost the bass I purchased them for... still waiting... high hopes for these ones, based on reviews they seem like my kind of flats.
Dunlop flats: Mainly interested in these because they have an exceptionally great selection of scales/gauges for a flatwound. Not sure I'm gonna like the sound though. From reviews it seems like they could be more suitable for the Chromes/Fender flats crowd. Which is fine of course, just not for me... or are they?
I'm sure lots of you will disagree with some (or all) of the above so I'll just say it one more time: IMO. YMMV.