Southfringe Sanctum is a little remote, being almost as far south as you can go past Helgen/Fort Neugrad, but isn't too cavernous and has plenty of storage, is well lit, and has 2 alchemy labs.
If you prefer something a bit darker and more northern, Harmugstahl is a cave/fort in the Reach near Dragonstooth Crater with a lab.
Those are your best bets for an easily accessible base, where the cave storage won't respawn if you're gone. There are some better and cosier caves but not really that many that don't respawn.