[CAST-128][1] has 64-bit block size. Such block size is obsolete. Don't use such block size (thus don't use CAST-128), unless you must.

[LOKI97][2] is broken, see Lars R. Knudsen & Vincent Rijmen's [_Weaknesses in LOKI97_][3]. Don't use it.

[RC5][4] with it's originally suggested parameters (64-bit block size, 128-bit key, 12 rounds) is broken, see Alex Biryukov & Eyal Kushilevitz's [_Improved cryptanalysis of RC5_][5]. Although that paper suggests parameters for the 128-bit block version, I'll pretend RC5 is obsoleted by [RC6][7].

This leaves [MARS][6] and [RC6][7] as competitors. Neither algorithm is close to broken AFAIK. They have been discussed and compared by NIST's [_Report on the Development of the Advanced Encryption Standard (AES)_][8]. RC6 is found to be faster, significantly so in hardware. RC6 is simpler in my view, but it has one drawback: it uses variable rotation, which is hard to make fast and constant-time on some low-end CPUs lacking a [barrel shifter][9] (including modern CPUs, e.g. some variants of ARM Cortex M0).

If I desperately needed a block cipher (ruling out a simpler stream cipher like [ChaCha][10]) and for some reason was facing the choice in the question (thus could not use [AES/Rijndael][11]), I'd probably use [MARS][6], unless speed was paramount and I knew for certain no CPU lacking a barrel shifter would ever be used (in which case I'd consider [RC6][7])

Addition: If I was stuck to a 64-bit block cipher among those of the question, I may use [CAST-128][1]: it's unbroken AFAIK, and even endorsed by [ISO/IEC 18033-3:2010][12], reviewed 2020.


  [1]: https://en.wikipedia.org/wiki/CAST-128
  [2]: https://en.wikipedia.org/wiki/LOKI97
  [3]: https://www.esat.kuleuven.be/cosic/publications/article-366.pdf
  [4]: https://en.wikipedia.org/wiki/RC5
  [5]: https://doi.org/10.1007/BFb0054119
  [6]: https://en.wikipedia.org/wiki/MARS_(cipher)
  [7]: https://en.wikipedia.org/wiki/RC6
  [8]: https://nvlpubs.nist.gov/nistpubs/jres/106/3/j63nec.pdf
  [9]: https://en.wikipedia.org/wiki/Barrel_shifter
  [10]: https://en.wikipedia.org/wiki/Salsa20#ChaCha_variant
  [11]: https://en.wikipedia.org/wiki/Advanced_Encryption_Standard
  [12]: https://www.iso.org/obp/ui/fr/#iso:std:iso-iec:18033:-3:ed-2:v1:en