For me it was Superion, 5 jets, that turned into robots and combined, awesomesauce. It was also my first combiner team.

a close second was Defensor, my only other combiner until I bought the reissue Predaking (actually no, I just remembered I bought a second hand Bruticus about 2010). Defensor was part of an epic laybuy I did in my early teens I think over a period of many months, occasionally having new transformers added to the pot before I finally paid it all off.