What's best/most stable/reliable hotkey spellcasting mod?
Best is subjective.
I use a combination of cyclers...most notably Hot Key Cyclers http://www.tesnexus.com/downloads/file.php?id=17402 . I use this for spells and weapons,
I also use ANB Item Switcher http://www.tesnexus.com/downloads/search.php . I use only the ammo switcher from this one for my arrows but it has a few others you may consider.
I was also using Super Hotkeys http://www.tesnexus.com/downloads/file.php?id=22326 . This mod can extend the number of hotkey wheels to whatever you want (unlimited and configurabl) and is very customizable. I don't use this this one currently only because with the above two I don't need it (Plus I had a habit of forgetting which "wheel" was active and it was pissing me off when I tried casting a heal spell and I was on the "wrong" hotkey wheel.)
All three of these are stable and I haven't had any issues with any of them in my game of 320+ mods.
One word of advice..read the included respective read me files for each to set up properly.