something that should be obvious to most people but in order to beat a Rammus you HAVE to have AP you can't take a good rammus out with AA also NEVER turret dive him under any circumstances.
The best tactic i have against a Riven player is to use
it will shut her down quick as long as you arent too far behind when you get it, and you want to harass her as much as possible then hope your all in does more than hers..
As a Varus main his biggest weakness is mobility and mana reliance early game so all in him at early levels so that he doesn't snowball because a fed varus is GG. lock him down early and do as much damage as you can when you know he has used up a lot of mana. usually in order for him to kill he has to have 3 blight stacks and an ability to detonate them. so no mana = free kill on him.
these are just my opinions, others may have better advice on how to shut these guys down but to my experience this works.