Singed does alot damage if used correctly, and if he desides to dive you, he will have you in the poison. 65flat ms from the ult, 75% slow on the W and his fling, if You have trouble staying away from garren, you have no chance staying away from singed, and he has just as good tank steroids as garen to tank towers.
As Maximum Morde said, rammus if used correctly is both harder to avoid and harder to ignore than garen, while beeing tankier.
Volibear has a 4sec 50% speedbuff on 8sec cd, garen has 3.5sec 30% speedbuff on 8sec cd. So saying garen is more mobile is utter bullshit. I'd also argue that voli is tankier if his passive is up, and he has far more cc to keep you in range.
Udyr has a 3sec 35% speedbuff on 6 sec cd, more base ms and up to 15ms from passive, so he is certainly more mobile than garen, and a 1sec stun >>>>>> 1.5 sec silence. his tankiness also is not much lower due to his low cd shield.
you just got beaten by garen and now your salty, that's all.