The fact that he let Phoebe fall in love with a demon and Belthazor was able to get close to them proved he was terrible as a whitelighter.
He's a better Whitelighter to his OTHER charges/NOT dealing with the Charmed Ones. He's a good Whitelighter in the following episodes:
S1E14 "Secrets & Guys" - Max's Father
S1E21 "Love Hurts" - Daisy
S2E4 "The Devil's Music" - Dealing with Carlton
S2E16 "Murphy's Luck" - Arresting the Darklighter
S4E1-2 "Charmed Again, Pt 1 & 2" - Dealing with Cortez
S4E17 "Saving Private Leo" - Maria
S5E4 "Siren's Song" - Melissa
S5E22-23 "Oh My Goddess, Pt 1 & 2" - Saving the entire world from the Titans
Otherwise, the only times I can think of where he was an actual good Whitelighter to the Charmed Ones was only in two episodes S4E5 "Size Matters" & S5E23 "Oh My Goddess, Pt 2". He was a better human post S7E16 "The Seven Year Witch" which is what I voted for. Him being a teacher/headmaster of Magic School was probably the best compromise, of both worlds, for him, to do via S7E19 "Freaky Phoebe" & S8E22 "Forever Charmed". Which funny enough, Leo was at his best as a "Whitelighter" (despite not being one), to Paige in S7E19 "Freaky Phoebe", than the rest of the show, but I digress.