persistent would be good for hideous laughter.
I vote for metamagic extend. This should give the largest benefit based on your spells; summon monster would be improved as an attach spell, and your utility spells would also be better. Of course, if you are looking to improve your attack prowess, another choice might be better.
I strongly disagree with the extend
Invisibility/blur/good hope is 3 minutes. Usually that will last long enough.
For summon monster 3 yes it is a touch more usefull, but not by much.
Forcing someone to save twice against your spells drastically improves their worth. If your saves are at like 16, then you are look at people with around 55% chance of failing the DCs. Reducing it to aroubnd 20%. If you have high DCs of like 18+ with a rod you drop the odds to like 10% of them with standing.
Why do only your current spells matter? You will gain more and possibly change some of the ones you have now with time.
Extend is just generally useful.
Bouncing and Persistent are good on save negates type spells, bouncing for the single target kind only, Persistent also good for ones that give new saves on later rounds (like hideous laughter).
Quicken is very expensive, but very nice if you can afford it.
Reach adds a lot of versatility.
Dazing, Empower, and Selective are also awesome rods, but I don't think a bard has much use for them. And Silence is great, but bard spells can't use it.
If your DM is letting you get ANY lesser rod regardless of cost, Quicken is kind of a no-brainer just for its normal price tag.
Have an opportunity to gain a lesser metamagic rod of whatever type ... With this bard spell list, what type would be most beneficial?
1st
Cure light wounds, touch of gracelessness, lock gaze, grease, saving finale, hideous laughter2nd: invisibility, blur, cure moderate, versatile weapon, sound burst
3rd:
Good hope, mad monkeys, summon monster three, dispel magic
I'll cast a vote for Quicken as well. Quicken Summon Monster so you aren't standing around for a whole round doing nothing, get the thing out and attacking right away, and still have a standard action to lay down a buff spell or some bardic music. On that note, you should really look at picking up Haste.
I'm fairly certain that Quicken rods (even though it's a lesser as far as rods go) is considered Medium treasure, which may be why this thread exists because why wouldn't you pick quicken?
If you end up swapping some spells to pick up more crowd control options that allow for saves, then Persistent would be rather good, but with your list now, it's Quicken all the way.
Also, I don't think you need to drop Summon Monster just because the Summoner has it: summoning more monsters is always a good thing, and it means that you two can switch off summoning to cover different needs.