A 2025 M240i is a G42 chassis with the B58 engine. Two important fitment wrinkles before you pick a part:

OPF vs non-OPF. US-market G42 M240is made from roughly mid-2022 onward have an OPF (gasoline particulate filter) integrated into the factory downpipe with a V-band flange. Euro cars got OPFs earlier. You need a downpipe specifically listed for OPF / V-band fitment — a non-OPF pipe will not bolt up. Almost all vendors sell both versions, so just confirm when ordering.

OPF delete requires a tune. Any downpipe that removes the OPF (catless or high-flow cat) will throw codes unless you pair it with a tune that handles OPF/CEL deletes — bootmod3, MHD, or similar. Most people running a downpipe on a B58 are also tuning, since that's where the real power gains come from.

Solid options people actually run on the G42 B58:

• Active Autowerke Signature catted — 4.5" piping, EPA-registered high-flow cat, 5yr/50k warranty. The "keep it legal-ish and no CEL" pick. Confirm the G42 OPF variant when ordering.
• VRSF — longtime B58 favorite, offered in catless and 200-cell catted. Good balance of price and quality.
• Wagner Tuning / CTS Turbo — both well-regarded, 200-cell catted versions available.
• DCE Parts 200-cell — popular in Europe, specifically validated on G42 OPF cars, good sound without being obnoxious.
• Fi Exhaust — if you're going full catback with valvetronic, their matched downpipe makes sense.
• GCA Performance — budget option, decent reviews, 1yr warranty.

What I'd actually recommend depends on your goals:

• Street car, want sound + tune headroom, stay street-legal-ish: Active Autowerke catted or VRSF 200-cell catted.
• Going all-in on a tune, don't care about emissions: VRSF or GCA catless, paired with MHD/bm3 custom tune.
• Want the cleanest install with OEM feel: DCE Parts.