Well, at least in March, it was Vegas. Seattle tends to overtake Vegas in the summer months, though. Here are (mainline only) passenger counts for March:

1099539 ORD
915006 DEN
602238 SFO
428846 IAD
425842 LAX
147658 LAS
119575 SEA
104571 MCO
104123 BOS
90279 SAN
85384 PDX
83096 HNL
80763 PHX
77320 LGA
58305 TPA
57155 LHR
56816 NRT
55436 BWI
53856 PHL
50297 MSP
47964 SNA
45630 DCA
45388 FRA
44892 EWR
43604 DFW
37372 OGG
36471 JFK
34288 SMF
31977 SJC
31195 OMA
31143 YVR
26922 SLC
26393 OAK
25589 FLL
25574 MSY
24934 BDL
24781 MCI
24607 MIA
24038 RNO
24012 KOA
22437 ATL
21431 HKG
19428 YYZ
18426 SJU
18401 ONT
18179 DTW
17526 IAH
17329 CUN
17283 SYD
17146 PEK
16994 CMH
16888 PVG
16095 PIT
15098 LIH
14774 CLT
13746 MEX
13205 KIX
12943 BOI
12462 ABQ
12422 PBI
12366 GEG
12309 SAT
12209 MUC
11832 TUS
11250 PVD
11190 CDG
11030 CLE
10848 AMS
10692 GRU
10597 DSM
10427 JAX
10146 PVR
9475 YYC
9182 TUL
8994 AUS
8614 OKC
8450 STL
8278 ICN
8236 BUR
7260 ICT
7164 BIL
6689 PSP
6019 JAC
5840 SJD
5216 EGE
5131 GRR
5113 ROC
5076 BRU
4951 MHT
4846 RIC
4741 CVG
4707 GUA
4510 EZE
4464 ZRH
4423 TPE
4248 NGO
4236 AUA
3919 BTV
3700 RSW
3502 SAL
3382 ANC
3071 BUF
2948 IND
2934 ALB
2914 HDN
2886 MEL
2786 DAY
2651 STT
2544 MBJ
2455 SXM
2354 SGN
2289 KWI
2202 MDT
2197 GIG
2078 BKK
2011 SIN
1621 GSO
1321 NAS
1218 ZIH
1154 PUJ
910 MVD
786 MTJ
633 LIR
413 GUC