5-10. Lower levels are often a great deal of fun but AS DM, they are more limited in adventure design. You run into that wall where the dice are still too fickle and can too easily turn fights into TPK's as readily as cakewalks. At 5th that is largely behind you, wizards begin to really come into their own in their spell power, variety, etc. (for use by pc's or against them).
At higher levels it really does start to get... complicated. Adventure design involves taking into account a LOT more PC capabilities, working up notably more details for NPC's and so forth. Again, you can have just as much fun - but AS DM, it also tends to be more work for the same reward. Even players tend to forget about items they have in their possession, spells or abilities they might use, etc. They settle on the same old tried-and-true (but also staid and dull) attacks and methods until it all goes kablooey and they NEED to scramble. But generally, they can be all-too-methodical. There are more and less optimum abilities and methods, yet attempting to always short-circuit those only feels arbitrarily manipulative - it becomes an annoyance rather than something that generates fun and interest.
I've never tried Epic and I doubt I ever will. I've run and played in campaigns to near-20th in 1e, 2e, and 3e and that has always been more than problematic enough for me.
