There are a lot of Cleveland fests I love, but I'll go with these for my top five (no particular order) based on my experiences...
Hessler Street Fair - http://ohiofestivals.net/?p=812
Larchmere Porch Festival - http://ohiofestivals.net/?p=940
Jak of All Trades Fest - http://ohiofestivals.net/?p=1191
Cleveland Asian Festival - http://ohiofestivals.net/?p=805
Tremont Greek Festival - http://ohiofestivals.net/?p=842
These other fests are fun too!
Clifton Arts and Music Fest - http://ohiofestivals.net/?p=917
Feast of the Assumption - http://ohiofestivals.net/?p=89
Romanian Festival - http://ohiofestivals.net/?p=92
IngenuityFest - http://ohiofestivals.net/?p=125
Brite Winter Fest - http://ohiofestivals.net/1-brite-winter-festival-revisited-cleveland-february-18-2012/