For my smithing characters it was rings.
I would join the Companions, then run off and explore in certain areas a couple of quests to get the transmutation spell and unlock a couple of gold, silver, and iron mines on the map.
I'd fast travel, fill my inventory with various ores, transmute them till all I had was gold, then forge gold rings and sell them to every merchant who'd by them from me. If I raised any given involved skill enough to level up, I'd immediately run for Kodlack and spend money on Blackmsith training till either I hit my training limit for that level.
Then wait until the mines respawned and start again.
I also did the quests to join the Orc Strongholds and cleared got the one with the Ebony mine on my map so once I got to Blacksmithing 100 I could easily forge and upgrade a set of Deadric Armor.
Nothng makes you feel as invincible as running around with a maxed-out armor ranking and the best possible unenchanted weapon for your preferred fighting style when the NPC enemies are still using iron.