Ratchet and Lockdown from Animated, kinda repeated in the movies

Bulkhead and Breakdown in Prime

Always saw G1 Bumblebee and Ravage as rivals, because they were both the small spies I guess

Blaster and Soundwave are usually set up as rivals and fought to the death in Japan

Many teams have a rival team combines definitely (Stunticons and Aerialbots, Combaticons and Protectobots, Technobots and Terrorcons) but also smaller teams (Triggerbots vs Triggercons, Sparkabots vs Firecons)

Apparently Sky Lynx was meant to be Predaking’s rival but don’t think that ever happened anywhere.

Same team rivals are interesting too, like Scourge and SkyByte in RID