What's the longest transcript known?
Top 10 long processed transcripts in humans (with multiple isoforms), from gencode 19 annotations:

Transcript    Length(bases)
------------------------
TTN-018        108861     &lt;-- Titin
TTN-019        103988      
TTN-002        101206     
KCNQ1OT1-001    91666
TTN-201         82413
TTN-202         82212
TTN-003         81838
MUC16-001       43732
TSIX-001        37026
MCC-009         29616


Ignoring isoforms (only longest isoforms shown)

Transcript    Length(bases)
------------------------
TTN-018         108861
KCNQ1OT1-001     91666
MUC16-001        43732
TSIX-001         37026
MCC-009          29616
TRAPPC9-015      29514
SYNE1-001        27602
GRIN2B-001       27204
OBSCN-011        26811
NEB-204          26020


Titin clearly is the longest transcript in humans

However this is the list of longest genes:

Gene           Length(Kb)
-------------------------
CNTNAP2        2304.64
LSAMP          2186.93
DLG2           2169.35
DMD            2092.29
PTPRD          2084.57
MACROD2        2057.83
CSMD1          2056.87
EYS            1987.24
LRP1B          1900.28
PCDH15         1806.76
CTNNA3         1783.65
ROBO2          1740.82
RBFOX1         1691.87
NRXN3          1619.64
DAB1           1548.83
RP11-420N3.2   1536.21
PDE4D          1513.42
FHIT           1502.09
AGBL4          1491.06
CCSER1         1474.33


Top 5 in Zebrafish (Zv9.75); longest isoforms:

ttna-203             93727   &lt;-- Titin
ttnb-202             82632
si:dkey-16p6.1-001   67263
syne2b-201           31867
si:dkey-30j22.1-001  29269


Top 5 in Drosophila (FlyBase r6.02); longest isoforms:

dp-RQ           71300   &lt;-- Dumpy
sls-RP          56448   &lt;-- Titin
Muc14A-RA       48719
Msp300-RG       43105
Ank2-RU         42107


Top 5 in C.elegans (WormBase WS220); longest isoforms:

W06H8.8g        55623   &lt;-- Titin
K07E12.1a.2     39257   &lt;-- dig-1
ZK973.6         25608
C09D1.1b        24198
C41A3.1         23457


Top 5 in Arabidopsis (TAIR 10.23):

AT1G67120.1     16272   &lt;-- Midasin homolog
AT3G02260.1     15451   &lt;-- Calossin-like protein
AT5G28263.1     15194
AT1G43060.1     14622
AT5G30269.1     14590


Top 5 in yeast (SGD):

YLR106C         14733   &lt;-- Midasin
Q0045           12884   &lt;-- Subunit I of cytochrome c oxidase
YKR054C         12279
YHR099W         11235
YDR457W          9807