They did build straight eights and Mercedes were the finest exponents of the art. Many pre-war GP cars were fitted with straight eights and Mercedes had the best of all. Alfa Romeo used a straight eight, as did Auto Union (now absorbed by Audi). I take it your mother has a 993?
The finest engine must surely be the V-12, and probably the finest V-12 is the Jaguar V-12 in 6.0 litre form. Normally aspirated: forced induction always seems to detract from engineering purity, hence the supremacy of BMW's M3 or M5 motors or, indeed, the V-12 they fitted to the McLaren F-1 (now that might pip the Jag).
As for interesting engines, how about the latest VW W-8 and Audi W-12? Bentley will apparently get a W-16 from the VW fold. Bugatti (also VW) will apparently utilise a 1001 bhp W engine, either 16, 18 or 24 cylinders, depending who you believe. This W configuration is in fact a double U format: two narrow angle V engines joined at the crank. A servicing nightmare!
Mercedes has, of course, threatened a 1000 bhp V-24 for the Maybach but this has apparently been shelved in favour of a less satisfactory twin turbo V-12.
1999 SL 500
1993 320 TE
1986 500 SEC