For regular T5 bulbs I can't help you..
As for T5HO bulbs, I've heard a good 2 bulb combo is an ATI Aquablue Special and an ATI Blue+ will give great par of any combo and a good color combo (cool whitish/blue). Others will likely suggest Geismann bulbs and other brands too which are great bulbs in themselves.
I'm currently trying to source the ATI bulbs myself but am having a hard time sourcing any. Lots of place have them in 36" and 48" but not 24". If anyone knows of any locally or where I can order them in Canada please let me know!
Here's a good site comparing the par produced by various bulbs from different manufacturers.
http://sites.google.com/site/tfivetesting/par