I'm gonna have to batten down the hatches Steve Bruce style for this one, but I'm saying Zinedine Zidane. Brilliant player and incredibly good to watch, but really inconsistent as an effective performer. He had plenty of full seasons when he didn't produce. In his entire career he only won 3 league titles, despite sharing most of it between an absolutely outrageous Juventus side and the Galacticos of Real Madrid.
If we look at his career pre-Juve, aged 23/24, he was still being rejected by Blackburn and Newcastle. Now, don't get me wrong, those clubs were clearly stupid not to take him (Jack Walker being quoted as saying "Why would we need Zidane when we've got Tim Sherwood?" is particularly hilarious. This point is, can you imagine anyone turning down Messi/CR7/R9/Ronaldinho/Neymar/Mbappe etc at 23/24?
Again, I'd reiterate that Zidane was a brilliant player; one whose ludicrously aesthetic style brought him a cult following. He also produced the goods on the biggest occasions (World Cup, Euros, Champions League final). Week to week, however, I'd argue that he was never consistent, which contributed to his star-studded clubs rarely winning the league, and never (!!) winning a domestic cup. For this reason, he's not an all-time great, or even among the very elite of my lifetime.