For me, it would be teleport. I find it boring to play around as it makes risky plays not worth doing/way more risky than needed. Makes people who don't know how to manage a lane properly and die when minions are at tower not lose as much as they should, or die and you start baron/teamfight and they can TP in later on in the fight/as it starts due to spawn times being really short now.
It just stalls out the game in ways that I find really boring and annoying to have to remember/keep tabs on, when if they took an offensive/defensive summoner for plays, the game would progress faster. I miss the old S1-S4 era where no one took TP and it was actually flash ignite top and X + ignite middle (sometimes tp, very rare). Now it's 1-3 tps a game despite the nerfs. It also ruins champion identifies such as twisted fate (hence why you don't see him anymore really).
I just find it unfun. And before you go "well then how do you outplay them?". Well, if they had an actual summoner instead of a map teleport summoner, they could fight back/defend and actually make a play with it and be safer. Same concept on why people are forced to take flash even if there's nothing good to flash besides an auto attack.
If you didn't take flash on a jungler, expect early invade-level 2-3 pressure by the enemy jungler and potentially the enemy laner who will shove and invade. Can't take smite + ignite or you'll be punished usually as you have no way to defend yourself if caught.
SO UH... yeah what summoner would YOU want removed?