What is the recomended sequence for signalling RBF?
The only other relevance of the nSequence field is as relative locktime (in version 2 transactions); see BIP 68.

If you do use relative locktime, they will determine the value of nSequence.

If you don't use that feature, any nSequence lower than 0xfffffffe will do.

In BIP125, there is no need for strictly increasing nSequence versions. instead, the highest fee wins.