Ah sorry: I think I understood your question only now.
All involved sides have commanders of the opposite side as priority No.1 on their targeting lists. But, the death of the same is not always changing situation on the battlefield. Or at best, it does, but only temporarily. Overall, it's changing next to nothing.
For example: instructed by the IRGC-QF, the regime launched this campaign very early during protesting, back in 2011, aiming to kill leading activists. It sniped, arrested, detained, tortured and murdered thousands of them - with no effect. They were always rapidly replaced by - usually - more skilled, more cautios too, people.
The insurgents hit back with a similar campaign, which reached its peak in July 2012 with the bombing of that HQ in Damascus, killing several of Bashar's top commanders. But, these were replaced by junior officers, and then by Iranians.
When the ISIS began spreading in Syria, it went after insurgent commanders and did its best to execute dozens of these, in turn throwing much of the insurgency into chaos, and bringing it on the verge of collapse. What happened: new, much more skilled commanders appeared, better supported from abroad (especially by Saudis), and more capable of re-uniting the insurgents.
Except for killing thousands of civilians (whether by targeting apartment buildings or bakeries), the SyAAF is running a campaign of targeting insurgent HQs at latest since summer 2012. At earlier times they were primarily deploying Mi-25s and L-39s for this purpose. For example, they have hit the same insurgent HQ in SE Aleppo four times within two days, killing a number of COs. Without effect, as can be seen.
Since October last year, the SyAAF is deploying MiG-23MFs armed with Kh-23 (AS-7 Kerry) and MiG-29s armed with Kh-29s (AS-14 Kedge) to target insurgent HQs in Aleppo and Idlib Provinces. One of MiG-23s has hit the HQ of the Liwa al-Tawhid in N Aleppo, back in December, killing the CO and most of the command cadre (a MiG-29 did something similar in Dayr az-Zawr, practically 'killing' a major onslaught on remaining regime positions in that city, and buying the regime plenty of time to recover, in return). As a consquence, Liwa al-Tawhid lost much of its integrity, and then influence too. But, overall, this didn't disturb the recover of insurgency in this part of Syria: they forced the ISIS out, and are now back in force in that city.
I guess that it shouldn't be too hard for the IASF to track down and kill Suleimani. It has already killed one of his deputies during bombardment of RGD bases in NW Damascus, back in February 2013. But, this changed absolutely nothing (or if, then it reinforced Suleimani's resolve).
Indeed, let's expand this 'Israeli example' even further: the Israelis are liquidating all the possible Arab leaders and military commanders already since something like 50 years (at least since they attempted to assassinate Field Marshal Amer, by downing the Ilyushin Il-14 that was to return him to Cairo from a visit to Damascus, on the evening of 28 October 1958). They were trying to kill Arafat for 30 years (some say they've eventually got him with a dose of Polonium), and they are violating every single cease-fire with Hamas by attacks on its leaders until this very day.
And they were never successful with this 'tactics'. It's rather amazing how insistent are Israelis at reinforcing an obvious failure: indeed, so amazing, that I really wonder why didn't they hit Suleimani long ago.
But overall, all of this is why I say that it's 'hard to gauge'. No matter how many and what commanders in Syria are killed, this war is a stalemate already since one year.
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