Highland Lake

I prefer this card over the Izzet Guildgate purely for flavor reasons: fond memories of playing Magic by the lakeside in the Berkshire Mountains, back when I first learned to play in 1994.

It slows any deck down, of course, but used properly it won't impact your tempo much. Don't have a turn-one move? Drop it first turn. Nothing going on Turn 2? Drop it then. Don't have a good 3 casting cost card to play on Turn 3? Well... sucks to be you, but go for it.