Well there are a few things that certainly help with keeping the player engaged in the battle system. Ultimately engagement is what you want regardless of how you choose to grab the player's attention.
Chrono Trigger is an amazing game and a total classic that people still love to play even after all these years. The combat system is very simple and the game itself is actually very easy. I'd say there are 4 things that Chrono Trigger really hits out of the park that made it such a hit. First, the combat isn't too hard numbers-wise so the player isn't required to grind to make it through the story. Second, each character has a small but diverse array of fun abilities with the ability to combine skills into dual-techs and triple-techs for all kinds of fun, powerful abilities. Third, the game puts battles directly on the world map so there are no transition screens and everything flows super nicely which makes the combat seem even faster. Fourth, the battles being on the map means that enemy positioning is now important and enemies move about during battle in real time, so the player is able to catch groups in AoE skills if they're patient.
Persona 4 has a pretty great combat system that makes the dungeon segments one of the best parts of the game. Battles take place on a separate battle screen, but are initiated on the map and the player can gain a free first turn if they ambush enemies or get ambushed if they aren't careful so they need to pay attention. There's a certain rhythm to combat since turns are taken one at a time instead of issuing orders to the party all at once so the player has the option to have party members respond to ally skills or alter their actions based on enemy moves. It's centered around exploiting enemy weaknesses to knock them down, but monster compositions often have a variety of weaknesses and resistances so the player has to consider their skills and turn order to either try and get all enemies knocked down at once to perform an "All Out Attack" to try and finish them all in one shot or focus an enemy at a time by targeted downed enemies to stun them and nuke them. The battles don't get boring because each "round" can go by very quickly and the player can get destroyed if they are careless and not paying attention to enemy resistances.
Combat doesn't need to be hard necessarily. Short combat encounters seem preferable to long ones unless you have a good reason to make them long. Boss battles of course can be total marathons as long as you vary it up with multiple stages so the boss changes things up partway. You also don't need an endless list of skills, a handful of fun ones works. One or two mechanics to center the player's attention on - something that is common to every battle which can make combat a test of the player's skill with that one mechanic - can also keep the player engaged on a turn-to-turn basis. Enemy position, enemy states, weaknesses, anything will work as long as it is consistent and relatively easy to understand.
Also having good effects for skills and a killer soundtrack goes a long way.
