IMHO, NONE of them! Now this is based on my experience with a Motorola RAZR V3m using Verizon Wireless as my carrier and running either the (older) NEWC_01.09.02 stock software, or my ViVo flashed NCR_01.18.00R software that I have now. Your results may vary if you have a different phone/provider/software.
I have read way too many postings in the forums here and elsewhere about people pretty much 'bricking' their phones by deleting these many (but very small) files to try and free more memory on their phone. The posts usually go along the lines of, 'my phone doesn't boot, PLEASE HELP', 'my phone no longer connects to my comp, PLEASE HELP', 'my phone just keeps re-booting, PLEASE HELP', etc. I haven't really taken a byte count, but I doubt that by doing so you'd gain even one megabyte of storage on your 23mB Razr. I would never delete ANY stock files from my phone, I would find it akin to trying to free hard drive space on my comp by randomly deleting files/folders from my c:\windows directory. That is my opinion based on my experience.
Now if you want to do so anyway, AND THIS IS OUTSIDE MY EXPERIENCE, you might be able to free a few bytes by doing so, but ONLY delete files that are NOT being used by your phone at all. Any stock ringtone/pictures or anything else that might be attached to one of your text messages/caller id's, or whatever else, you MIGHT be able to delete. BUT again, in my honest opinion, why go through all that trouble to make sure the file you want to delete is not 'pointed' to by your phones f/w, if all you might gain is enough for a few more text messages or one more ringtone?