I hate to be cynical about it, but I'm sure every home game against the Phils this year will rank among the top attended games.
In theory perhaps, but not in practice - in fact, the 13,413 attendance for the Phillies' visit on 12th April of this year is our lowest turn-out this season (thanks no doubt to goshawful weather)...also, our tenth-lowest turn-out of last season was the 14,309 who went on Monday 27th September of that year.
Here are all the attendances for Phillies visits, excerpted from my complete list (hence the constant repetition of 'vs Phillies'):
41,290 Monday 5th April 2010 vs Phillies [season/home opener; highest of 2010 and 4th-highest of all time]
40,386 Monday 13th April 2009 vs Phillies [home opener, 5th-highest of all time]
38,049 Saturday 31st July 2010 vs Phillies
35,807 Sunday 1st August 2010 vs Phillies
34,789 Monday 30th May 2011 vs Phillies [Memorial Day]
34,039 Tuesday 29th July 2008 vs Phillies
32,590 Friday 30th July 2010 vs Phillies
31,798 Wednesday 30th July 2008 vs Phillies
31,658 Thursday 31st July 2008 vs Phillies
29,577 Sunday 17th May 2009 vs Phillies
28,393 Monday 1st September 2008 vs Phillies [Labour Day]
28,105 Tuesday 20th May 2008 vs Phillies
28,055 Wednesday 21st May 2008 vs Phillies
27,240 Wednesday 7th April 2010 vs Phillies
25,394 Monday 19th May 2008 vs Phillies
24,875 Thursday 14th April 2011 vs Phillies
24,495 Wednesday 1st June 2011 vs Phillies
23,896 Saturday 16th May 2009 vs Phillies [Game 2]
23,430 Friday 15th May 2009 vs Phillies
23,150 Tuesday 2nd September 2008 vs Phillies
23,122 Wednesday 3rd September 2008 vs Phillies
21,017 Tuesday 31st May 2011 vs Phillies
20,494 Thursday 16th April 2009 vs Phillies
20,217 Thursday 8th April 2010 vs Phillies
20,026 Wednesday 29th September 2010 vs Phillies [home finale]
19,910 Saturday 16th May 2009 vs Phillies [Game 1 – make-up of rain-out]
19,117 Tuesday 28th September 2010 vs Phillies
18,706 Thursday 10th September 2009 vs Phillies
17,153 Tuesday 8th September 2009 vs Phillies
16,914 Wednesday 13th April 2011 vs Phillies
16,818 Wednesday 9th September 2009 vs Phillies
14,309 Monday 27th September 2010 vs Phillies
13,413 Tuesday 12th April 2011 vs Phillies [lowest attendance of 2011 thus far]
...so a good-sized crowd is usually, but not always, guaranteed.