No bootloader is the recommended option (you do still need to do "burn bootloader" to set the clock speed to what you selected - clock source is controlled by the "fusebytes" which are only written when you "burn bootloader" not on upload)
Micronucleus is used for the boards that have a USB port for direct upload, usually sold as "Digispark" (they are all chinese copies now). It uses VUSB (bitbang USB) to upload without an external programmer, but takes about 1/4th of the flash. VUSB doesn't work on all USB ports, but if it works at all, it is reliable.
Optiboot is only there for people who very much want to upload with a serial adapter. It takes about 8% of the flash. It works with any serial adapter, but is not 100% reliable, a power glitch or reset at the wrong time will break it. (I was going to say more, but it wouldn't survive google translate, and most people wouldn't understand it regardless of what language i wrote it in anyway!)
Re: t85 in general, a nano or pro mini is a little bit larger than an ATtiny85 >.> and all the Arduino boards have stuff like the power LED and regulator on board that waste power and are unsuitable for battery-operated projects. Besides, it's more fun to use a smaller processor. Throwing a nano at everything is boring!
On ATTinyCore as discussed, there's a version of Wire that should "just work" (though someone recently reported a bug that I do need to fix, Wire.readBytes · Issue #617 · SpenceKonde/ATTinyCore · GitHub ).