how hard is it to pledge top tier?

I'd argue theres only one true top-tier fraternity (PhiSig). They always have alot of pledges but there are so many people rushing it could be hard to find your way in. Tier matters to an extent but theyre not as defined as one would think. PhiSig, kinda SigEp SAE and FIJI are top with Sig Chi close, Sig Nu is on the rise I guess but most frats dont like them at all, Beta is a really great group of people but not considered top, LXA flies completely under the radar but they have their own following and are generally a great group of guys, and DTD is the bottom feeder. Id say Phi Sig is top, DTD is bottom and the middle is loosely Sigep SAE FIJI Sig Chi Sig Nu Beta LXA.

What about sororities?

sororities look for new members who will fit in with them and that they can have fun with. every sorority is different in their own ways, and while a lot of girls argue that some chapters are better than others, it doesn't really matter what "tier" you're in as long as you're in a sorority you feel comfortable in and actually like. but honestly, tier doesn't really matter to a majority of girls already in sororities as well as the rest of the school.

For sororities, tier only matters for pretentious PNM's. During recruitment, these are the girls who say things like "Yeah, Sorority A was really nice, but they're bottom tier, so I'd never accept a bid from them" or "I'll only accept a bid from Sorority B, C, or D. No one else." These girls are also the girls who wind up either not getting bids or getting bids from a sorority that isn't "top tier" and not accepting the bid. PLEASE don't become one of these PNM's.
Girls who are actually in sororities don't care about tier anymore. If you are happy with your sisters, no one cares what sorority you're in. I know girls who accepted bids from "top tier" sororities and are now miserable because they are nothing like their sisters. I also know girls who were hesitant to accept a bid from a "bottom tier" and now love their sorority to pieces. Once recruitment is over, tiers disappear. It solely becomes you enjoying (or not enjoying) the time you spend with your sisters. All sorority girls at Villanova, no matter what "tier" they are in, have similar experiences. Every sorority: mixes with the frats and sports teams,
has close-knit pledge classes,
knows how to have a good time,
has three major functions per semester (oftentimes, at the same venues),
gets invited to frat formals (very fun, overnight trips to places like AC)
and, most importantly, becomes a valuable part of each members Villanova experience.